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

close #3531 added focus handling and removed the badly announced "times" close button from screenreaders

move from downloads.html to customize.html to better match purpose of page, fix up customize page layout to match other docs, link style refinement to jumbotrons

remove popoverarrows mixin, relegate code to tooltips and popvers since we only used them once each; this fixes the problem of broken tooltips in 2.1

adding getting started page based on Bootstrap University from awhile back, testing out side nav on docs instead of subnav bar