Add llms.txt documentation support

[vgvoleg]

What changed

• Added sphinx-llms-txt to documentation dependencies.
• Enabled sphinx_llms_txt in the Sphinx config.
• Configured generated llms.txt links to use the deployed GitHub Pages base URL.
• Added a short LLM-ready Documentation section to the docs landing page linking to llms.txt and llms-full.txt.

Why

This publishes LLM-friendly documentation entry points similar to other Sphinx-based projects, while keeping links valid for the repository's GitHub Pages deployment path.