Add llms.txt generation for LLM-friendly documentation

[shaanmajid]

Closes #1498

Generates llms.txt and llms-full.txt for the documentation site. Also generates markdown files at each path (e.g., /configuration/index.md), enabling LLMs to fetch raw markdown instead of HTML.

As mentioned in the issue, this wasn't natively possible, since Zensical doesn't yet support MkDocs's plugin ecosystem. I built llmstxt-standalone to work as a standalone llms.txt generator, respecting the config for mkdocs-llmstxt.

• Configure llmstxt plugin in mkdocs.yml with section structure
• Add docs-build and docs-compile mise tasks
• Update CI to run llmstxt-standalone after zensical build

Note: the generated files are not served or visible when using zensical serve, but will be generated and published to docs site as part of the publish-docs workflow.

Preview:

• llms.txt
• llms-full.txt

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.56%. Comparing base (5c64e47) to head (3951a85).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1553      +/-   ##
==========================================
- Coverage   91.56%   91.56%   -0.01%     
==========================================
  Files          91       91              
  Lines       17875    17875              
==========================================
- Hits        16368    16367       -1     
- Misses       1507     1508       +1     

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[j178]

Thanks!