Add flexbox-based equal-height grid columns as an experimental example

Based on @pete-otaqui's work from #11851.
Per @mdo's feedback in that issue, this makes it an experiment rather than integrating the feature into Bootstrap itself.
Also renamed the class to `.row-eq-height` and removed the screen size variants, as I don't quite get what the use-case is for those (and this just keeps things simple).

(An actual screenshot image of the example needs to be added.)