- Add/replace callout for color and AT with include

"Go somewhere" is more descriptive of an example link, rather than a
button that triggers in-page functionality. Also makes it consistent
with all other examples in this page

These can be replaced by their `.text-xs-*` parallels.
This also avoids any complications from interactions between the responsive and non-responsive classes.
(e.g. `<div class="text-left text-md-right">`)

Refs #18300
[skip sauce]

resolves #17609

[ci skip]

Fix #17182

Otherwise the "Heads up!" is at the end of the previous line, making the connection to the warning unclear.

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