docs: publish llms.txt and llms-full.txt for agent-friendly ingestion (#18276)

Two machine-readable entry points to the Hermes Agent docs:

  /llms.txt         curated index of every doc page, one link per page
                    with short descriptions. ~17 KB, safe to load into
                    an LLM context window.
  /llms-full.txt    every page under website/docs/ concatenated as markdown.
                    ~1.8 MB. For one-shot ingestion by coding agents and
                    RAG pipelines.

Both files are also served from /docs/llms.txt and /docs/llms-full.txt
(Docusaurus serves website/static/ under baseUrl=/docs/). Some agents and
IDE plugins probe the classic site-root path; the deploy workflow now copies
both files to _site root so either URL works.

Conforms to the emerging llmstxt.org spec: H1 project name, blockquote
summary, short install command, GitHub link, then curated sections
mirroring the docs-site navigation (Getting Started, Using Hermes,
Features, Messaging, Integrations, Guides, Developer Guide, Reference).

Generated by website/scripts/generate-llms-txt.py. Wired into prebuild.mjs
so every 'npm run build' and 'npm run start' refreshes the files alongside
the existing skills.json extraction. Both outputs are gitignored (same
precedent as src/data/skills.json).

Descriptions in llms.txt are pulled from each page's frontmatter, so they
stay current automatically. All ~80 section slugs are validated against
the filesystem at generation time; an invalid slug would fail the prebuild.

website/.gitignore

@@ -8,6 +8,8 @@
 .docusaurus
 .cache-loader
 src/data/skills.json
+static/llms.txt
+static/llms-full.txt
 
 # Misc
 .DS_Store