tweak grid docs copy

[skip sauce]
``aria-label="close"`` instead of sr-only ``span``

shorter, neater, well supported
Make sidebar navigation actual ``<nav>``
Expand advice about adding labels

Harmonizing this with same advice given for CSS inline forms
``<abbr>`` with title also useful for AT

...and remove the redundant paragraph for basic abbreviation, which
simply repeats what was just said above.
Merge pull request #15320 from patrickhlauke/docs-accessibility-tweaks

Docs accessibility tweaks: use ``aria-label`` for close ``<button>``, expand advice for ``<abbr>``, form control labels, make sidebar navigation a ``<nav>``
fixes #15311: mention in docs requirement for .btn-group-* sizing on immediate .btn-group
Add missing to
Merge pull request #15323 from BBosman/for

Re-add missing "to" in docs
Expand advice for optional icons with hidden labels

to clarify/hint that a ``<label class="sr-only">`` is not the only way
to do it
Split out/clarify ``<select>`` and ``multiple``
Callout for labelling of input groups

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)
Use aria-label for pagination prev/next

Cleaner alternative to using ``<span class="sr-only">``
Make main segmented button keyboard-focusable

unless there's a reason why ``tabindex="-1"`` was dropped in there...
grunt-saucelabs now uses `throttled` instead of `concurrency`

[skip validator]
Merge pull request #15327 from patrickhlauke/docs-accessibility-tweaks2

More docs accessibility tweaks
tweak browser bug list order so Safari bugs are contiguous
tweak browser bug list order so Chrome bugs are contiguous
Add grunt-contrib-compress.

Now, the zip file is created when running `grunt prep-release`.

[ci skip]
Update glob.