docs(docs-site): add llms.txt for docs site

[François (Lp-Francois)]

Summary

• add a spec-compliant llms.txt file to docs-site/public/ so the Astro docs site publishes it at the site root
• point the file at existing raw markdown sources for the README, API, architecture, spec, host reference, and example skills
• verify the docs site build emits docs-site/dist/llms.txt

See https://llmstxt.org/ to read more about why the llms.txt file (without having to install a skill, people can point their agent to the link https://contentful.github.io/skill-kit/llms.txt.)

Verification

• pnpm build (in docs-site/)

[François (Lp-Francois)]

Merging #73 instead