BS3: Navbar overhaul

Conflicts:
	dist/css/bootstrap.min.css

fix mixin code example

BS3: Drop the accordion for the panel (fixes #8648)

Conflicts:
	dist/css/bootstrap.min.css

Added pagination-size mixin

Added input-size mixin

BS3: `.btn-group`s for your `.btn-group`s

Fixes #9415

patch highlighting hint

* Deletes the accordion.less file
* Extends the panel to be collapsible with our JS plugin
* Supports panel color variations

Emphasize git HEAD downloads

Conflicts:
	components.html
	dist/css/bootstrap.min.css

add HTML5 validation of docs pages to the build

* Now responsive by default (fits better with mobile-first approach)
* Requires `.nav-header` to group `.navbar-brand` and `.navbar-toggle`
for proper mobile display.
* Changed `.nav-collapse` to `.navbar-collapse`
* Simplified examples in docs