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.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/source/_toctree.yml

@@ -191,6 +191,28 @@
 - sections:
   - local: contributing
     title: Contribute to LeRobot
+  - local: writing_docstrings
+    title: Writing docstrings
   - local: backwardcomp
     title: Backward compatibility
   title: "About"
+- sections:
+  - local: api/robots
+    title: Robots
+  - local: api/teleoperators
+    title: Teleoperators
+  - local: api/cameras
+    title: Cameras
+  - local: api/motors
+    title: Motors
+  - local: api/datasets
+    title: Datasets
+  - local: api/policies
+    title: Policies
+  - local: api/processor
+    title: Processors
+  - local: api/envs
+    title: Environments
+  - local: api/configs
+    title: Configuration
+  title: "API Reference"