This also fixes#21280. Previously, we were using float and clear on the individual controls, but that meant a computer height of `0` for the parent `.custom-controls-stacked`. No more problem after using flexbox though.
- we need to set margin for space between footer buttons as flexbox doesn't render HTML spaces like inline-block does
- flexbox doesn't have collapsing margins or anything, so we hack it with some :not(:first-child) and :not(:last-child) action
- 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.
Given move to flexbox, we can make this available for everyone and clean it up some.
- Simplifies the `.nav-justified` styles to build on the flex-powered `.nav` base class.
- Adds the `.nav-fill` to make nav links fill available horizontal space, but not equal width.
- Set an explicit `display: flex` on the base `.nav` class and remove the floats/clearfixes from our navs.
- Set some global styles for `.nav-link`, a departure from the previous solution that featured no predefined padding.
- Move `.nav-link` from `inline-block` to `block` given this setting was added for our tabs and pills anyway.
- Remove horizontal margin from nav tabs; let folks set that on their own from now on.
