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(reported by ianhernandez in Slack)

Related to #17314.

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Mostly links.
Fixes #17056.

Fix #17125

- 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

- Replaces manual use of .bd-callout with {% callout [type] %}
- Rearranged some callouts for proximity to others
- Turned long lists of callouts--like those on tooltips, plugings, etc--into a list because holy shit that's overwhelming

Per https://github.com/paypal/bootstrap-accessibility-plugin#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin