sure we only need the two mixins, but this ensures folks don't run into missing mixin errors down the line

* Don't show `bd-*` classes to the user

* Add comments and another regex

Fixes #21287

plus shx? 

remove broken example class from grid docs

follow up to #21436 to add docs for the .flex-row-reverse , .flex-column-reverse, and .flex-wrap-reverse

Use pixels instead of rem units for focus box-shadow on custom controls so that IE11, Edge, and Win FF screw it up less

Fixes #21135.

There's a height mismatch here because we're setting padding on the labels, while inputs get padding *and* a top+bottom border. This now uses calc to determine the exact height needed to match labels to inputs.

Rather than overriding this just on inline form examples, this should be for all static form controls to match inputs.

fixes broken inline form example mentioned in #21407

This also fixes #21280. Previously, we were using float and clear on the individual controls, but that meant a computer height of `0` for the parent `.custom-controls-stacked`. No more problem after using flexbox though.

- 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)