Add llms.txt for htmx.org

[ryan-blunden]

Closes #3642

Adds www/static/llms.txt so the site serves https://htmx.org/llms.txt.

What changed

• added a concise, manually maintainable llms.txt file
• organized it around the requested sections: summary, key docs, reference docs, examples, essays/concepts, source repo, and guidance for LLMs/coding agents
• kept the initial version intentionally curated rather than exhaustive

Verification

• cd www && zola build
• confirmed www/public/llms.txt exists
• zola serve
• curl -fsS http://127.0.0.1:1111/llms.txt
• confirmed served content matches www/static/llms.txt
• npm run lint
• npm run types-check
• npm run test