docs: restructure README into collapsed framework sections

- Reorganize into 4 collapsible sections: Tailwind CSS, Panda CSS, StyleX, CSS function
- Add intro explaining two Tailwind setup options (CSS import vs JS plugin)
- Remove embedded StyleX Vite config; link to official StyleX docs instead
- Make "install or copy/paste" section framework-agnostic
- Replace "integration path" terminology with neutral "option" phrasing
- Update TOC and requirements to reflect multi-framework support

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dogmar