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

* [Fix #19849] Tabs are opened even if disabled.

* fix hund code review hints

* rollback hound issues because Travis fails

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

- 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.