- Improve feature flag callouts
- Add API details to migration notes and clarify examples
- Restructure guides to use sub-heading structure instead of snippet tabs
    - Offers ability to explain differences between APIs more thoroughly
    - Also a better experience, because users' tab preference will not apply across snippets
- Clarify example snippets
    - Only demonstate one concept per snippet
- Adjust heading levels
- Add globals API reference section
- Improve snippet filenames
    - Remove redundant `storybook-` prefix
- Prose tweaks

- Explain globals API and feature flag once, at the top
- Reference that callout in consistent manner throughout page
- Wrap entire docs section in "feature flag callout" to better differentiate
- Remove headings for with/without feature flag sections
    - Headings will be removed in SB 9+, so they're not a stable reference

- Polish and details
- Make Viewport match structure and style of Backgrounds

Co-authored-by: jonniebigodes <joaocontadesenvolvimento@gmail.com>