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