Give all embedded live docs examples data-example-id attrs

Refs #11410
Merge pull request #15214 from twbs/example-ids

Give all embedded live docs examples data-example-id attrs
[skip sauce]
Remove execute permission on fonts.
Update source maps info in README.md.
Merge pull request #15305 from vsn4ik/fix_fonts_permission

Remove execute permission on fonts.
Merge pull request #15306 from vsn4ik/whats_included_readme

Update source maps info in README.md.
Repeat warning comment about keeping "What's included" sections in sync

Refs #15238
[skip sauce]
Customizer: Catch and properly report compile errors that Less.js's toCSS() can throw.

Currently these errors are only reported in the JavaScript console, not in the Customizer UI.
Refs #15296

[skip sauce]
[skip validator]
Merge pull request #15309 from twbs/fix-customizer-compile-err-reporting

Catch and properly report compile errors that Less.js's toCSS() can throw
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``