- add more details on custom checkboxes and radios for how we handle the states and how the css works
- create a disabled custom checkboxes and radios example

Based on v3 http://getbootstrap.com/css/#forms-help-text, adapted to
make examples more "real life"

[ci skip]

[skip sauce]

Reunites the family.
Closes #17791

[ci skip]

Use the `row` class on the `form-group`, and change the `control-label` class to `form-control-label`

Mostly links.
Fixes #17056.

- 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

- Fixes #13267 somewhat.
- Instead of a single, block-level class, let's use a combination of existing elements and classes.
- For block-level help text, use p.text-muted.
- For inline-level help text, use span.text-muted or small.text-muted.

- 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