Docs: Refactor to use .less file instead, importing Bootstrap vars and mixins, and cleaning all the things up

This way, it works in all browsers.

The validator doesn't choke anymore on this and even though it's not spec compliant, it's needed for some IE versions. So until we move favicon.ico in root, this should do the job.

Closes #13379.

X-Refs:
https://github.com/twbs/bootstrap/commit/b2a67ffe55f5fedb0ae1d5dff331687c3e744be0
https://github.com/twbs/bootstrap/commit/c97e6d5cad74d20257d28734f85311ef5d00b560

The Google Analytics account we use cannot currently be upgraded to Universal Analytics. In order to keep using Analytics, we have to stick with Classic for now.

- works when viewing page via file://
- sets a good example security-wise

...because we don't support it and it may cause problems for people
trying to view the docs in IE.

In HTML5 in tag script, the type attribute is no longer required. The default value is "text/javascript".
http://www.w3.org/TR/html5/scripting-1.html#attr-script-type