Fixes #11572: Add contextual background-color classes to match text classes; move both sets of classes to Helper Classes section of the CSS docs

I probably missed a few things and screwed up a few others, but this
takes the place of much of #9910. I haven't added many variables—that
should be reserved for v3.1—but I have rejiggered the layout, added
missing ones, corrected wrong values, etc.

/cc @cvrebert @juthilo @saas786

fixes #10526: Remove custom background on responsive tables and set it in the docs where it should've been originally

* Fixes unrelated typo in readme
* Add new Welcome section with history, team, callouts, and community
subsections
* Redesigns callouts to be a bit lighterweight with colored headings