bump up height of progress bars, give some extra depth with stacking bars, and then make the stacked bars actually work

Merge branch 'progress-multi-2.1.0-wip' of https://github.com/jpommerening/bootstrap into jpommerening-progress-multi-2.1.0-wip

fix static navbar container issues, clean up docs presentation by unfixing the top navbar in favor of static one, start adding table of contents for longer sections of docs

fixes #4044: make z-index of tooltips same as fixed navbar so they overlay it if generated from within

fixes #4444: clarify docs copy that new icon white style is intentional on active and hover links in .nav and .dropdown-menu

fixes #4445: .btn-link shouldn't have a pressed button-like state at all, so nuking everything on :active as well