Update RL module API docs

[yuecideng]

Description

The embodichain.agents.rl module API documentation was not displaying any
classes or functions. This fix adds explicit documentation for each RL
submodule so that Sphinx can properly generate the API reference.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.

[Copilot]

Pull request overview

Fixes the Sphinx API reference page for embodichain.agents.rl by explicitly documenting RL submodules so their public classes/functions appear in generated docs.

Changes:

• Removes a malformed/indented module title line so the embodichain.agents page renders correctly.
• Adds explicit .. automodule:: sections for embodichain.agents.rl.{algo,buffer,models,train,utils} to surface RL APIs in the docs.

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[Copilot]

The section underline for "Policy Models" is shorter than the title text. While Sphinx/docutils often tolerates this, it can lead to incorrect section parsing in some configurations. Consider extending the underline to match the full title length for robustness.

Suggested change

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[Copilot]

The section underline for "Utilities" is one character shorter than the section title. To avoid any reStructuredText section parsing issues, make the underline length match the title length.

Suggested change

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