feat: add llms.txt endpoint for LLM-optimized documentation

Add /llms.txt endpoint that serves a concatenated, text-only version
of all Tailwind CSS documentation pages optimized for Large Language
Model consumption.

- Extract text from MDX files, removing JSX components and preserving
  code blocks
- Remove standalone HTML blocks (not in code blocks)
- Extract meaningful content from custom components (ApiTable,
  ResponsiveDesign, etc.)
- Statically generate the output at build time
- Include all 185 documentation files in proper order with sections

Author: quantizor