add HTML5 validation of docs pages to the build

use `data:` URL of empty invalid PNG: src="data:image/png;base64,"

Make docs HTML more valid

fix ID of <select> in fieldset[disabled] example

fixes #9258: no more float bug with full-width grid columns by removing float on all .col-*-12 classes

undo the grid max-width boundary because it makes offsets kinda borked (you'd have to set twice, for medium and large, in most cases--and that sucks)

social-buttons.html: drop 'px' from height+width attrs per HTML5

fixes #9392: no need for width auto on inline/navbar form controls given inline-block on parent form group

_includes/social-buttons.html: escape ampersands in URLs so HTML validates

Instead of applying `.btn-` or `.input-` to individual elements in
button and input groups, we now have new sizing classes to keep your
markup a little leaner.

* Add `.input-group-sm` or `.input-group-lg` to your `.input-group` to
replicate `.input-sm` and `.input-lg`, respectively.
* Add `.btn-group-xs`, `.btn-group-sm`, or `.btn-group-lg` to your
`.btn-group` to replicate `.btn-xs`, `.btn-sm`, and `. btn-lg`,
respectively.

Both sets of new sizing classes simply use the existing individual
sizing classes as mixins, so customization of them is automatically
inherited when compiling.