Refactor index.html: add README links, fix cursor styling, remove dead code

- Add cursor: default to non-clickable github-label elements
- Add 3 main README links (Main, Frameworks, Scenarios) in headers
- Remove ~90 lines of unused .github-link CSS rules
- Replace inline styles with reusable CSS classes
- Add .readme-link styling with document icon and external link indicator

Author: Tom-Dynamsoft