Merge har.fyi under docs

[max-ostapenko]

Summary

Part 3 of splitting #1276 into sequential PRs (based on #1328 courageous-galliform).

This PR merges har.fyi content directly into httparchive.org under the /docs/ path:

• Adds Astro Starlight integration (@astrojs/starlight, starlight-links-validator, starlight-llms-txt)
• Migrates all guides, reference docs (tables, blobs, structs, custom metrics, functions), and assets into src/content/docs/
• Integrates custom Starlight header matching HTTP Archive branding and search
• Updates sitemap and timestamp generation to index doc pages
• Updates site navigation and references pointing to har.fyi to relative /docs/ paths

Sequence

1. Migrate from Python to JS (staying on AppEngine) #1311 (migrate-astro-appengine into main)
2. Lazy load CWV distribution histogram on user click #1328 (courageous-galliform into migrate-astro-appengine)
3. This PR (merge-har-docs into courageous-galliform)
4. Firebase Hosting migration (next PR into merge-har-docs)