docs: expose llms.txt via docs.page redirect

[leoafarias]

Summary

• Add docs/llms.txt.mdx to expose /llms.txt on docs.page and redirect to the canonical raw llms.txt.
• Update docs CI to run on llms.txt changes and verify the redirect target is set correctly.
• Update README and docs index with explicit human-docs and agent-context endpoints.

Validation

• npx -y @docs.page/cli check

[docs-page]

To view this pull requests documentation preview, visit the following URL:

docs.page/btwld/ack~84

Documentation is deployed and generated using docs.page.