Update CSS Best Practices

[dmtrmrv]

Description of the Change

This PR completely restructures and modernizes the CSS best practices documentation (_includes/markdown/CSS.md and css.md) to provide more comprehensive, actionable, and modern guidance for writing maintainable CSS.

Benefits:

• More actionable guidance for developers at all levels
• Better alignment with modern CSS development practices
• Improved maintainability and scalability guidance
• Enhanced accessibility considerations throughout
• Clear migration path for adopting new CSS features

Verification steps:

1. Review the updated table of contents in css.md matches the new section structure
2. Verify all internal anchor links work correctly
3. Check that code examples are syntactically correct and follow established formatting
4. Ensure the mobile-first and accessibility principles are consistently applied throughout

How to test the Change

1. Build and serve the Jekyll site locally:

   bundle install
   bundle exec jekyll serve
   

2. Navigate to the CSS section (/css/) and verify:

   • All navigation links in the sidebar work correctly
   • Internal anchor links jump to the correct sections
   • Code examples are properly formatted and highlighted
   • Content flows logically from general principles to specific implementation details

3. Cross-reference with the main index page (/) to ensure:

   • CSS section links are updated and functional
   • The updated navigation structure is reflected in the homepage listing

4. Validate content accuracy:

   • Review code examples for correctness
   • Ensure all external links are functional
   • Verify that the guidance aligns with current CSS best practices

Changelog Entry

Changed - Completely restructured and modernized CSS best practices documentation with updated methodologies, modern CSS features guidance, and enhanced practical examples

Credits

Props @dmtrmrv @joesnellpdx @dainelmawer @Antonio-Laguna

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [ ] I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• [x] All new and existing tests pass.