doc: add generate-library skill

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request adds a new skill guide (SKILL.md) outlining the end-to-end process for generating and validating new C++ client libraries from Buganizer requests. The feedback suggests replacing a hardcoded reference to biglake_quickstart with a generic <library>_quickstart placeholder to maintain consistency throughout the documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (48eeb77) to head (e721aef).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16396      +/-   ##
==========================================
- Coverage   92.26%   92.24%   -0.03%     
==========================================
  Files        2246     2246              
  Lines      212121   212121              
==========================================
- Hits       195717   195661      -56     
- Misses      16404    16460      +56     

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