docs(processor): bring src/lerobot/processor/ to 100% docstring coverage

Documents every remaining public class/function across the 15 files in
lerobot.processor (pipeline, batch/device/normalize/rename/observation
steps, env-specific and HIL steps, action bridges, tokenization,
language, converters, factory functions, and the normalization
migration script), reformats pre-existing docstrings to the
machine-checked Args:/Returns:/Raises: standard, and expands
docs/source/api/processor.mdx from 3 documented classes to the full
public surface.

Adds lerobot.processor to check_docstrings.py's MODULES_TO_CHECK
ratchet and removes the module's ruff D-ignore. Docstrings and doc
comment reformatting only; no behavioral changes.

utils/check_docstrings.py

@@ -60,6 +60,7 @@ PATH_TO_LEROBOT = PATH_TO_REPO / "src" / "lerobot"
 # Modules whose public objects are checked. Add a module here once its docstrings follow the standard.
 MODULES_TO_CHECK = [
     "lerobot.robots",
+    "lerobot.processor",
 ]
 
 # Objects that do not yet follow the standard, so the check can be green from day one. Removing an entry