docs: add llms.txt via mkdocs-llmstxt plugin

[bearomorphism]

Description

Add auto-generated llms.txt and llms-full.txt for the docs site using the mkdocs-llmstxt plugin.

Instead of a hand-crafted static file, the plugin auto-generates both files from mkdocs.yml on every build, keeping them in sync with docs automatically. llms-full.txt includes expanded page contents for direct LLM consumption.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (GitHub Copilot CLI)

Generated-by: GitHub Copilot CLI following the guidelines

Code Changes

• Add test cases to all the changes you introduce (N/A - docs config only)
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

After mkdocs build, both site/llms.txt (index) and site/llms-full.txt (expanded contents) are generated automatically from the mkdocs.yml plugin config.

Steps to Test This Pull Request

1. Run uv run mkdocs build --strict
2. Verify site/llms.txt and site/llms-full.txt exist
3. Check that llms.txt contains grouped links matching the mkdocs.yml nav

Additional Context

Part of #1936. The SKILL.md portion is addressed in PR #1949.

Switched from a hand-crafted docs/llms.txt to the mkdocs-llmstxt plugin based on research showing that auto-generation keeps docs in sync and also produces the commonly-expected llms-full.txt companion file.

[Copilot]

Pull request overview

Adds an llms.txt static asset to the MkDocs documentation site to provide an agent/LLM-friendly index of key Commitizen docs pages (grouped by topic), supporting discoverability for automated tooling.

Changes:

• Add docs/llms.txt with grouped, canonical links to major documentation sections (overview, commands, configuration, tutorials, plugins, contributing).

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[bearomorphism]

Validation

Verified that all review comments have been addressed:

• Grammar fix: 'teams that enforces' -> 'teams that enforce' ✅
• Grammar fix: 'Managing tags formats' -> 'Managing tag formats' ✅
• PR scope: Changed 'Closes Provide a skill for agentic coding #1936' to 'Part of Provide a skill for agentic coding #1936' since this PR only covers llms.txt; SKILL.md is in PR docs: add Agent Skills SKILL.md #1949 ✅
• Build: uv run mkdocs build --strict passes, site/llms.txt is generated correctly ✅
• URLs: All doc links verified against mkdocs.yml nav structure ✅
• llms.txt spec: Follows the format from https://llmstxt.org/ (H1 + blockquote + H2 file lists) ✅

Currently researching whether the file placement at /commitizen/llms.txt (GitHub Pages subpath) is discoverable by LLM tools in practice. Will update with findings.