- 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

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