Alternate fix to #17965: Updates to use the new class name and adds some basic docs for custom checkbox/radio validation

Fixup for 365db6e3
/fyi @mdo
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Closes #19049

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Put all the textual input types right next to each other for easy visual comparison.
Refs #17308, #18763

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It has been removed from the HTML specification: https://github.com/whatwg/html/issues/336
Only Presto Opera ever implemented a special UI for it.

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- 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"

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Reunites the family.
Closes #17791

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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.