Fixes #18493
Closes #18836
[skip sauce]

And link to a relevant blog post w/ more info.
Refs #16966
[skip sauce]

- Remove nested > selectors for dropdown items
- Update docs examples to use all the new markup

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

- Replaces manual use of .bd-callout with {% callout [type] %}
- Rearranged some callouts for proximity to others
- Turned long lists of callouts--like those on tooltips, plugings, etc--into a list because holy shit that's overwhelming