docs: add llms.txt outputs to docs build

[danielscholl]

What changed

• Updated docs/build.js to generate _site/llms.txt
• Added _site/llms-full.txt generation
• Added markdown companion output at page-aligned paths (*.html.md)
• Reused shared frontmatter stripping helper across build steps
• Cleaned llms.txt snippet extraction for more readable descriptions

Why

Enable docs output to support llms.txt-style LLM discovery with:

• a curated index (llms.txt)
• a full corpus export (llms-full.txt)
• markdown companions aligned to rendered page URLs

Testing notes

Built docs locally:

node docs/build.js --out _site

Verified outputs:

• _site/llms.txt
• _site/llms-full.txt
• _site/**/*.html.md

[bradygaster]

Thanks for this - I'm excited to see if this helps. If you have ideas on how we could test it before/after I'm all about it.