feat: add hand-crafted llms.txt with build-time validation

[crtahlin]

Summary

Closes ethersphere/DevRel#838.

• static/llms.txt — Hand-crafted, llmstxt.org-compliant index file with 75 curated doc links organised into 8 task-oriented sections (install, configure, upload, host, build, concepts, reference, optional). Includes intro context (chunks, DISC, xBZZ, ports) and top-level resource links (API ref, OpenAPI spec, bee-js, Book of Swarm, whitepaper).
• scripts/validate-llms-txt.mjs — Prebuild validation script that cross-checks llms.txt URLs against actual doc files. Reports stale links and missing coverage as warnings (exit 0, non-blocking).
• package.json — Chains the validation script into the existing prebuild hook.
• README.md — Documents the llms.txt workflow and what to do when validation warnings appear.

Also includes the plugin config fix from b62582c that reconfigures docusaurus-plugin-llms to only generate llms-full.txt.

How it works

npm run build
  → prebuild: fetch-awesome-swarm.mjs && validate-llms-txt.mjs
  → docusaurus build (generates llms-full.txt via plugin)
  → static/llms.txt copied to build/ as-is

Every build warns if llms.txt has drifted from the actual docs, without blocking.

Test plan

• npm run build succeeds, validation script runs with only expected warnings (4 intro pages + 1 stub)
• npx docusaurus serve — /llms.txt returns the hand-crafted file, /llms-full.txt returns plugin output
• Spot-check 5–10 URLs from llms.txt to confirm they resolve (not 404)
• Temporarily rename a doc file, run build, confirm the script warns about the stale link

🤖 Generated with AI

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[darkobas2]

And who is gonna check the logs and fix ?

[crtahlin]

If you have any suggestions to use some channel for alert messages, if that is possible? In any case, one of the files is manually generated, so checks just alert if they find discrepancies. It can and should be checked like the other files, in the "human" docs, for perhaps being obsolete.