to harmonise it with the callout given in pagination

...as per the text immediately preceding this example

Fix #17234

[skip sauce]

- 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

/cc @cvrebert @patrickhlauke

- 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

Remove Wall of Browser Bugs entry.
Update mention of the bug in the CSS docs.
[skip sauce]