* Add some flexbox nav components
  - Includes .nav-justified for inline, tab, and pill nav components
  - Includes example of using the flex utils (.d- and .flex-items-) for centered nav
* redo heading hierarchy a bit

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=510079
Closes #21120

[skip sauce]

Small grammar fix.

And move one Firefox entry to keep the entries sorted by browser.
Refs #21089

Refs #21089
[ci skip]

These would help in making hidden skip links show up visually once they
receive AT focus (for sighted/partially sighted AT users)

* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin

* make spacer utils responsive by grid tier
* update scale to add two levels, document them
* change responsive spacing utils to avoid the xs abbreviation in the class name for that tier
* update code snippet to match source
* update usage in our docs
* linter
* docs updates

floated content has to come first in the DOM, otherwise you can get alignment bugs. in flexbox though, that's not the case. as such, i'm adding this example code to guide folks to the changes needed to move between default and flex modes.