Assorted accessibility (and some consistency) fixes for documentation

Closes #14951 by merging it.
Fix Issue #15076.

This commit adds documentation explaining the need to have an initial
.active element in carousels.
Improved accessible glyphicon advice/use

...to actually make it consistent with what I said in the glyphicon docs
section...
aria-hidden added to some stray glyphicons

plus minor tweak to customizer's generated alerts
Merge pull request #15085 from patrickhlauke/glyphicons-in-forms-tweak

Glyphicon tweaks
Fixes #14939: Don't remove focus outline from buttons in button groups
Merge pull request #15077 from jayanthkoushik/issue-15076-fix

Fix Issue #15076.
Merge pull request #15086 from twbs/btn_group_focus

Don't remove focus outline from buttons in button groups
Wash-up of some stray dropdown examples

adding explicit aria-expanded="false" initial state (so they're
correctly announced by AT as being collapsed), and - in the case of
old-school use of <a> - role="button"
Merge pull request #15087 from patrickhlauke/dropdown-accessibility-tweaks

Wash-up of some stray dropdown examples