/cc @cvrebert

Mostly links.
Fixes #17056.

Example:
> ... SCSS variables (e.g., `@enable-gradients: true`) ...
Referenced SCSS but used the LESS syntax. Changed to:
> ... SCSS variables (e.g., `$enable-gradients: true`) ...

Closes #17114

Fix #17062

- 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