✨ feat(home): replace MkDocs homepage with standalone landing page

Strategy: use static docs/index.html as the entry point — completely
isolated from MkDocs theme, so visual changes cannot affect docs pages.

Landing page includes:
- Particle starfield background with periodic logo morphing
- Full-viewport hero with gradient title + big CTA to quickstart
- Scroll-down sections: Why Nexa, Harness 6-tuple, Community
- Stats bar: 23+ features / 1,935 tests / 16 examples / 20x AVM

Nav changes:
- Removed old index.md + index.en.md
- '快速入门' moved to top of 语法指南 section
- MkDocs docs now start at /quickstart/

extra.css, fonts.css, main.html: COMPLETELY UNTOUCHED (1353 lines)

Author: ouyangyipeng