Remove backface-visibility from progress bars

Fix form in navbar

* Bring back the navbar up top
* Pull the docs content back into separate pages because a 6.6k line docs file is just crazy
* Recenter the page content and bring back affixed side nav (needs work)

BS3: Require .btn-default for all standard buttons

* Overhaul the form control sizing section to only show sizing via grid columns as parents, not as classes on inputs
* Restore the inline-form option
* Restore the bottom margin on form controls and make them block level instead of inline-block
* More misc docs cleanup for forms

* Instead of , use  for standard, gray button
* Improves ability to customize buttons by encouraging folks to not override .btn and thus overriding all button modifier classes
* Updates docs to reflect the change
* Reorganize buttons CSS