Fix #17166

[ci skip]

- 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

- no real need for everything to be 100% shared
- padding looked and felt too large for inputs but not for buttons
- tying forms and buttons seems fine, but throwing in pagination feels wrong