Auto-generate llms.txt at build time

[AndreaRaccagni]

Replaces the hand-written llms.txt approach with build-time
generation using starlight-llms-txt
(github.com/delucis/starlight-llms-txt — Starlight core
contributor, MIT, actively maintained).

Generates three files at build time, all derived from the
actual Starlight pages:

• llms.txt — slim index pointing to the full content
• llms-full.txt — full documentation as markdown
• llms-small.txt — minified variant for smaller LLM context windows

[vercel]

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