- Add/replace callout for color and AT with include

Dump the type margin overrides in alerts for utils, document that particular example, sidesteps part of #18884

Since `aria-label` is used, it's not necessary anymore.

- 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