feat(llms): add llms.txt generation via sphinx-llm

[timkpaine]

Wire NVIDIA's sphinx-llm extension into the docs build so projects can emit llms.txt / llms-full.txt alongside the HTML output.

• pyproject: add llms extra (sphinx-llm>=0.4.1) and enable it for yardang's own docs via [tool.yardang.llms]
• build.py: load [tool.yardang.llms] config and pass use_llms plus the llms_txt_* passthrough options into the conf template
• conf.py.j2: register the sphinx_llm.txt extension and emit llms_txt_* settings when enabled
• tests: cover config loading, extension wiring, and default-off behaviour

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.55%. Comparing base (f066aeb) to head (e13dbb5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
+ Coverage   82.66%   83.55%   +0.88%     
==========================================
  Files           9       10       +1     
  Lines        1373     1447      +74     
  Branches      145      148       +3     
==========================================
+ Hits         1135     1209      +74     
  Misses        198      198              
  Partials       40       40              

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