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.

pyproject.toml

@@ -449,7 +449,6 @@ ignore = [
 "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/rollout/**" = ["D"]