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

Refs #11410
Update Wall of Browser Bugs entry for #14972

http://crbug.com/175502 is fixed in Chrome 40 and has been closed.
But Windows is still broken due to http://crbug.com/438064 ,
  which is open and currently targeting Chrome 41.
Whitelist <li role="separator">...</li>

Refs https://www.w3.org/Bugs/Public/show_bug.cgi?id=27120
Refs #15263

[skip sauce]
Merge pull request #15269 from twbs/role-separator

Whitelist <li role="separator">...</li>
[skip sauce]
automatic grunt dist
Temporarily revert "automatic grunt dist" so Grunt bot will run again.

This reverts commit 2c1c885ed4978900fb2a5e3310a428fd72b89c12.
automatic `grunt dist`
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.