Redesign DEVGUIDE: full CSS, home page, navbar, sidebar fix

- Add all theme CSS (sidebar, nav, typography, code, tables, dark mode) to application.css
- Fix toc-type.html: move nav before article so sidebar renders on left
- Redesign home page: hero section + 11 topic cards with icons + appendix strip
- Redesign navbar: 7 desktop links + grouped mobile menu with Data Formats/Protocols/More sections
- Update index.md to use layout: home

Author: ronaldtse