refactor(index): remove balloon tooltips and clean up HTML structure (#271)

- Remove balloon tooltip CSS (~210 lines) and all data-balloon attributes
- Remove unnecessary wrapper span elements from sample entries
- Combine duplicate external link icon CSS for readme/source links
- Update CSS selectors from :has(span) to :has(a) after span removal
- Simplify search JS to use text content only (aria-label removed)

Author: Tom-Dynamsoft