docs: add API documentation infrastructure (#4348)

* docs: add API documentation infrastructure

LeRobot's documentation build passes `--not_python_module`, which tells
doc-builder there is no importable Python package and disables `[[autodoc]]`
entirely. The result is that all 90+ pages are hand-written guides and there is
no generated API reference at all.

This is the machinery to change that. It deliberately contains no docstring
changes of its own — every docstring edit lives in the follow-up PR, so this
one can be reviewed as tooling and configuration alone.

**The standard.** `docs/source/writing_docstrings.mdx` is the contract: Google
section headers with Hugging Face type formatting, the machine-checked argument
line, `**Attributes**:`, doc-builder cross-references, fenced doctest examples.
It also records three behaviours that are not discoverable from the source and
were verified against a local build: `[[autodoc]]` silently skips members with
no docstring; doc-builder does not inherit docstrings from base classes, so a
registered config shim whose body is `pass` renders every field with no
description; and module-level aliases resolve to the canonical class.

**Autodoc turned on**, with two changes that are not obvious:

- `--version main` on the main-docs job. Without `--not_python_module`,
  doc-builder resolves the version from `lerobot.__version__` and only maps it
  to the default branch when it contains "dev". transformers relies on that;
  our main carries 0.6.2. Verified by building both ways — dropping the flag
  alone would publish the main docs to /lerobot/v0.6.2/ instead of
  /lerobot/main/ and disable notebook building.
- `pre_command` on both jobs. doc-builder ships a mock-deps registry entry for
  lerobot, so the reusable workflow takes its light-install path, which cannot
  import the package. The heavy dependencies cannot be mocked either: draccus
  runs `register_subclass` at import time and `processor/converters.py` calls
  `functools.singledispatch.register(torch.Tensor)`, which needs a real class.
  `[dataset]` is the only extra required.

Workflow triggers gain `src/**`, since the reference is now generated from
docstrings. `docs/source/api/` is excluded from the prettier hook, which reads
`[[autodoc]]` member lists as lazy paragraph continuations and joins a ten-entry
list onto one line.

Nine API reference pages, scaffolded with each module's base class.

**Doctests.** `LeRobotDocTestParser` is mandatory rather than optional here:
ruff's `docstring-code-format = true` drops the blank line before a closing
fence, after which stdlib's `_EXAMPLE_RE` reads the fence as expected output and
every example with output fails. It is written against the installed pytest
rather than copied from transformers, whose version predates pytest 9's
`import_path` signature and its own fix for the `@property` line-number bug.
`preprocess_string` also diverges: the upstream fenced-block split puts a
single-line example's code in a chunk with no `>>>` in it, so neither the CUDA
skip nor the `+IGNORE_RESULT` injection fires for it.

**Checkers.** `utils/check_docstrings.py` is the ~300-line core of the
2203-line transformers original; the `@auto_docstring` system, modular
propagation, GitPython and `checkers.py` are not ported.
`utils/check_config_docstrings.py` checks that every registered robot config
documents its port and calibration semantics.

**Gates**, all set to values that pass today: ruff `D` with per-file-ignores
per unconverted module, `interrogate` at `fail-under = 52` against a measured
52.1%, and Makefile targets wired into the quality workflow. The doctest
allowlist ships empty and the `doctest` target handles that, because the files
carrying runnable examples arrive with the docstring PR.

* ci: build the docs on Python 3.12

The shared doc-builder workflows create their virtualenv with the runner's
system Python, which is 3.10.12 on ubuntu-22.04. lerobot requires >=3.12, so
the build died during "Setup environment":

    × No solution found when resolving dependencies:
    ╰─▶ Because the current Python version (3.10.12) does not satisfy
        Python>=3.12 and lerobot==0.6.2 depends on Python>=3.12 ...

That step runs before `pre_command`, so the real install this workflow already
performs never got the chance to run. There was no fix available on the caller
side either: `env:` does not propagate into a reusable workflow, so `UV_PYTHON`
is unavailable, and `uv venv` runs in the runner workspace root rather than the
checkout, so a `.python-version` file cannot reach it. The non-light fallback
(`uv pip install "./pkg[dev]"`) fails identically, so this is not specific to
the mock-deps path — it blocks any package requiring 3.12+.

huggingface/doc-builder#808 adds a `python_version` input to both build
workflows, which this passes. Pins move to that merge commit, picking up three
unrelated fixes in the same range (#810, #811, #812); the upload workflow is
unchanged there and is bumped only to keep all three pins on one SHA.

* chore: sort imports in vla_jepa tests

Enabling pydocstyle in the previous commit changes how ruff determines where a
module's import block ends, which makes I001 fire on three vla_jepa tests that
were clean before. The blank line between the `conftest` and `lerobot` imports
is the trigger: both are first-party, so isort wants them in one contiguous
block, and the docstring-aware analysis is what makes it notice.

These files are unrelated to the API reference, so the fix is only to satisfy
the new gate.

pyproject.toml

@@ -401,7 +401,7 @@ exclude = ["tests/artifacts/**/*.safetensors", "*_pb2.py", "*_pb2_grpc.py"]
 # N: pep8-naming
 # TODO: Uncomment rules when ready to use
 select = [
-    "E", "W", "F", "I", "B", "C4", "T20", "N", "UP", "SIM" #, "A", "S", "D", "RUF"
+    "E", "W", "F", "I", "B", "C4", "T20", "N", "UP", "SIM", "D" #, "A", "S", "RUF"
 ]
 ignore = [
     "E501", # Line too long
@@ -411,9 +411,53 @@ ignore = [
 ]
 
 [tool.ruff.lint.per-file-ignores]
-"__init__.py" = ["F401", "F403", "E402"]
+"__init__.py" = ["F401", "F403", "E402", "D104"]
 # E402: conditional-import guards (TYPE_CHECKING / is_package_available) must precede the imports they protect
 "src/lerobot/scripts/convert_dataset_v21_to_v30.py" = ["E402"]
+
+# D (pydocstyle) is enabled globally, but only holds for code that has been converted to the docstring
+# standard in docs/source/writing_docstrings.mdx. Every module below is still on the old style; each entry
+# is deleted as that module is converted, and this block can be removed once it is empty.
+#
+# Not part of the API reference and not planned for conversion: tests, examples, benchmarks, templates,
+# CI helper scripts and the packaging shim.
+"tests/**" = ["D"]
+"examples/**" = ["D"]
+"benchmarks/**" = ["D"]
+"scripts/**" = ["D"]
+"setup.py" = ["D"]
+"src/lerobot/templates/**" = ["D"]
+# Vendored from transformers; keeps its upstream docstring style so syncs stay clean.
+"src/lerobot/policies/molmoact2/molmoact2_hf_model/**" = ["D"]
+# Awaiting conversion, one PR per module.
+"src/lerobot/annotations/**" = ["D"]
+"src/lerobot/async_inference/**" = ["D"]
+"src/lerobot/cameras/**" = ["D"]
+"src/lerobot/common/**" = ["D"]
+"src/lerobot/configs/**" = ["D"]
+"src/lerobot/data_processing/**" = ["D"]
+"src/lerobot/datasets/**" = ["D"]
+"src/lerobot/distributed/**" = ["D"]
+"src/lerobot/envs/**" = ["D"]
+"src/lerobot/jobs/**" = ["D"]
+"src/lerobot/model/**" = ["D"]
+"src/lerobot/motors/**" = ["D"]
+"src/lerobot/optim/**" = ["D"]
+"src/lerobot/policies/**" = ["D"]
+"src/lerobot/processor/**" = ["D"]
+"src/lerobot/rewards/**" = ["D"]
+"src/lerobot/rl/**" = ["D"]
+"src/lerobot/robots/**" = ["D"]
+"src/lerobot/rollout/**" = ["D"]
+"src/lerobot/scripts/**" = ["D"]
+"src/lerobot/teleoperators/**" = ["D"]
+"src/lerobot/transforms/**" = ["D"]
+"src/lerobot/transport/**" = ["D"]
+"src/lerobot/utils/**" = ["D"]
+"src/lerobot/lerobot_types.py" = ["D"]
+# Package root: two one-line docstring fixes land with the docstring PR.
+"src/lerobot/__init__.py" = ["D"]
+"src/lerobot/__version__.py" = ["D"]
 [tool.ruff.lint.isort]
 combine-as-imports = true
 known-first-party = ["lerobot"]
@@ -457,21 +501,24 @@ default.extend-ignore-identifiers-re = [
     "seperated_timestep",
 ]
 
-# TODO: Uncomment when ready to use
-# [tool.interrogate]
-# ignore-init-module = true
-# ignore-init-method = true
-# ignore-nested-functions = false
-# ignore-magic = false
-# ignore-semiprivate = false
-# ignore-private = false
-# ignore-property-decorators = false
-# ignore-module = false
-# ignore-setters = false
-# fail-under = 80
-# output-format = "term-missing"
-# color = true
-# paths = ["src/lerobot"]
+# Docstring coverage gate. `fail-under` is a RATCHET, not a target: it is set just below the currently
+# measured coverage so it passes today, and is raised in the same PR that documents a module. Never set it
+# to a value that fails on main. The destination is 100; see docs/source/writing_docstrings.mdx.
+[tool.interrogate]
+ignore-init-module = true
+ignore-init-method = true
+ignore-nested-functions = false
+ignore-magic = false
+ignore-semiprivate = false
+ignore-private = false
+ignore-property-decorators = false
+ignore-module = false
+ignore-setters = false
+fail-under = 52
+output-format = "term-missing"
+color = true
+paths = ["src/lerobot"]
+exclude = ["src/lerobot/policies/molmoact2/molmoact2_hf_model"]
 
 # TODO: Enable mypy gradually module by module across multiple PRs
 # Uncomment [tool.mypy] first, then uncomment individual module overrides as they get proper type annotations