Add documentation skill for manual review

[melissawm]

Description

This PR, previously discussed with the team, adds a Gemini skill to review documentation pages according to the Google Technical Writing style guide and specific MaxText guidance previously written.

The skill will not run automatically. Those interested in using it can activate their Gemini CLI from the MaxText repo and run something like:

/documentation-style review docs/install_maxtext.md

The skill will suggest changes that help with consistency, formatting and other general guidance, including docstrings. For example:

/documentation-style review docstring in src/maxtext/checkpoint_conversion/to_huggingface.py:main

Tests

Tested locally for both narrative documentation and docstrings.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• [N/A] I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[bvandermoon]

Does this only work with Gemini CLI? Or does it also work with Antigravity/internal equivalents?

[melissawm]

I am pretty sure skills are general enough to be used by any tool, but unfortunately I don't have access to those tools so I'm not able to test it.