- Overhauls the states, including the link/button variants, for list groups to better match how we handle .btn states.
- Moved the .list-group-item-action styles before the .list-group-item so that we don't have to do as much overriding.
- Removed the plain-hover-focus mixins from the disabled and active states since they're unnecessary.
- Added support for :active states on the .list-group-item-action variant (for the current click state).
- Removed the heading and text classes and variables since we can accomplish the same thing with utilities.
- Added support for :disabled on the .list-group-item-action as well since we can use those with button elements.
- Rearranged docs to reflect all the above changes.
- Reformatted some Sass variables.
- Adds new flexbox.md file to utilities docs
- Adds a `breakpoints.yml` data file for easier output of responsive classes in the docs. Will put this to use on other pages soon.
- Adds hella flex utils. There are some dupes for now, but they'll get removed in time.
- Reorganize things, leading with the base nav first instead of an accessibility note, and then moving the rest to a new available styles section.
- Add horizontal alignment options thanks to new flex utils (these new utils will be documented later in another commit).
- Dropped `.nav-stacked` for a flex util and added additional alignment options, so document those, too.
- better copy and examples throughout
- callout on flexbug #12 for inline elements like links and images (which is the problem in #20408)
- add order section
