- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.

it's important, yo

- remove screen reader stuff in favor of explicit labels
- remove 4 and 5 items to keep examples simpler

frees us up to not have to encourage so much markup throughout these pagination examples

- 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.

updates the dom order in our docs to match (floated content comes first, but no need for that in flexbox)

fixes #21423 by updating docs to mention flex display instead of inline-block; clarifies follow-up text about use of the proper utilities

- better table of contents and overall structure

- clearer examples of modal contents, longer modals, and modal options

- 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.