[Chore] llms.txt support

[Xmaster6y]

What does this PR do?

Key insights about the PR.

Linked Issues

• Closes #?
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Checklist

• I have read the CONTRIBUTING guide.
• I have added tests for my changes if needed.
• I have updated the documentation if needed.

---

Summary by cubic

Adds llm.txt support to the docs by installing sphinx-llm and enabling the sphinx_llm.txt extension in Sphinx. This builds a single, LLM‑friendly text file from our documentation.

• Dependencies
  • Added sphinx-llm@>=0.3.0 (docs extra); pulls in sphinx-markdown-builder@0.6.10 and tabulate@0.10.0.

^{Written for commit e057ce8. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (60ca555) to head (e057ce8).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

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##             main      #47   +/-   ##
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  Coverage   97.19%   97.19%           
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  Files          39       39           
  Lines        2426     2426           
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  Hits         2358     2358           
  Misses         68       68           

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[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Safe documentation-related change: adds a Sphinx extension for LLM-friendly output and its associated dev dependencies.}

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

_{Auto-approved: This is a low-risk documentation configuration change that adds LLM-friendly text support via a Sphinx extension.}