Upgrade to Bootstrap 5.3

[chalin]

• BS 5.3 changes are significant enough that, IMHO, we'll want to careful walkthrough of the 5.3 migration details before upgrading Docsy to BS 5.3.
• [Older comment] After we've released and sufficiently vetted Upgrade to Bootstrap 5.2 #470, we should upgrade to 5.3. Color modes have changed, and might help address some Docsy feature requests.

Tasks that we might be able to address at the same time:

• Support adding theme colors #1845

Base & prep

• Upgrade to Bootstrap 5.3.3 #1563

Tasks

Color modes

• Color modes replace dark variants for components, so .btn-close-white, .carousel-dark, .dropdown-menu-dark, and .navbar-dark are deprecated - [BS5.3] Replace .navbar-dark by data attribute and other style adjustments #1896

Utilities

• Deprecated the .text-muted utility and $text-muted Sass variable. It’s been replaced by .text-body-secondary and $body-secondary-color - [BS5.3] Replace text-muted by text-body-secondary #1892

Related

• Last PR preview using BS 5.2: https://deploy-preview-1886--docsydocs.netlify.app/
• https://github.com/twbs/bootstrap/releases/tag/v5.3.3
• Upgrade to Bootstrap 5.2 #470
• BSv5.2 upgrade followup #1510

[chalin]

Done AFAICT. Closing. Will reopen if anything comes up.