Add tabindex="-1" to main #content to work around bug in Chrome (and old
WebKits) and IE
Update advice in accessibility section
Include patch in docs CSS to suppress outline when #content is focused
(particularly as Chrome sets focus as result of mouse click for
tabindex="-1" elements)

Surprisingly those weren't reported by the validator; only in the grunt-contrib-htmlmin branch they were reported, so after digging into it, I saw they were indeed errors...

fixes #15497: increase decimals on line-height-large from 1.33 to 1.3333333 to fix button sizing in Win 8.1 Chrome

Additional markup for the skip link (to ensure it has the same effective
width as the main content, and to provide an outline around just the
link text) plus some basic styles...make it look a bit more in keeping
with the overall look and feel of the docs.

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

[skip sauce]
[skip validator]