Require .btn-default for regular buttons

* 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
Merge pull request #7809 from twitter/bs3_adding_btn_default

BS3: Require .btn-default for all standard buttons
Fixes #7816: revert negative margin on .navbar-nav
hide entire navbar in print views
Drop typeahead.js--instead try Twitter's typeahead plugin
Super massive docs overhaul

* 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)
move social buttons to separate include
refactor homepage jumbotron masthead doodad
make restored top navbar collapsable
red navbar for docs
new docs page headers
more navbar and header tweaks
more work on the sidenav, including small css refactor and new classes
docs css reorg
.bs-docs-header to .bs-header
.bs-docs-top to .bs-top
nuke stuff from customizer
.bs-docs-social-* to .bs-social-*
.bs-docs-footer to .bs-footer; nuke .bs-docs-container css; remove .bs-docs-container from footer
.bs-docs-example to .bs-example