rems: update breadcrumbs to use new sizing; improve variables by renaming @breadcrumb-separator to @breadcrumb-divider; change up colors

Added @gray-base variable to make @gray-{shade} reference, also allows @gray-base to be modified with less#modifyVars and the vars to cascade

In Sass, variable must be declared before it is used.
This also removes Miscellaneous section, because horizontal line color
and form / list paddings are Typography.

`@gray-light` darkens from #999 to #777, which puts `.text-muted` at the
threshold for the 4.5:1 WCAG minimum contrast[1]. #777:#fff is 4.48:1.

The “Example” headers in docs become #959595, which is contrast ratio
3:1, the minimum for larger text. Since the headers are less important
than the surrounding text, 3:1 is fine and an improvement on the
previous #bbb:#fff (1.92:1).

Fixes issue #13847.

[1] http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html