add flexbox to the not supported list 

https://github.com/twbs/bootstrap/issues/17802

[ci skip]

Refs #17237
[skip sauce]

Mostly links.
Fixes #17056.

Fix #17055

- 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