- Add .text-muted for text color control over .bg-inverse in the examples
- Add a dark navbar collapse example

Alternate fix to #18865: Change the .navbar-toggler color for light/dark navbars as we do with the .navbar-brand

v4: Card improvements

Here we're doing some margin swapping, so it looks a little funky. All this does is match the margin implementation and rendering across our table and flex versions of card decks.

- Remove background-color from header and footer in inverse cards (fixes #19841)
- Update border override for header and footer in cards; only need to declare a new color
- Update the selector for targetting blockquote footers in cards

- Remove the box-shadow and switch back to border to match .card basics
- Update the header nav margin override--since we restored the border, we need that default negative margin at the bottom

- Add inverse examples for all variants
- Use rgba() colors for hover and accent states for easy usage in inverse styles

Update tags/labels/badges migration docs to mention class and change in behavior for alignment in list groups

v4: Figuring out split dropdown buttons

- Really we didn't need to do any of that strip units stuff to generate six new variables, two for each button size
- Using sibling selectors, we can target those split button dropdown toggles using the .dropdown-toggle-split class, and adjust padding and margin as needed
- Now, we nuke the margin-left from the ::after generated caret and tighten up the padding so that those split toggles don't look huge next to their main button

v4: Forms cleanup