feat: LLM compatibility improvements & algolia update

[EchinopsisM]

Summary

LLM Compatibility

• Add description frontmatter to all documentation pages for richer search/LLM context
• Convert absolute /docs/... links to relative .md file links for better LLM navigation
• Add hand-crafted static/llms.txt index following the llmstxt.org standard
• Add scripts/validate-llms-txt.mjs build-time script that cross-checks llms.txt against actual doc files
• Configure docusaurus-plugin-llms to generate /llms-full.txt at build time
• Add prebuild hook to run validation automatically on every build

Analogous to the changes made in bee-docs (#779, #784).

Algolia Update

• Update Algolia config to point at a new crawler which now uses the full featured docsearch, leading to many more hits for searches (almost double on the few terms I tested)

How it works

```
npm run build
→ prebuild: validate-llms-txt.mjs (warns on stale/missing links, exits 0)
→ docusaurus build (generates /llms-full.txt via plugin)
→ static/llms.txt copied to build/ as-is
```

Test plan

• `npm run build` succeeds, validation script runs with no warnings
• `/llms.txt` returns the hand-crafted file
• `/llms-full.txt` returns plugin-generated full text
• Spot-check relative links in converted doc pages resolve correctly