Rename docs/LICENSE-DOCS to docs/LICENSE

It sounded weird.
refactoring: add & use .text-emphasis-variant() mixin
refactoring: add & use .bg-variant() mixin
add & use @modal-md Less var for uniformity
fix #12277

Thanks to @Quy for pointing these out.
present browser compatibility info in a table
copy
Move docs third-party libraries to a separate folder.
Fix typo in index.html
Make spelling of Less uniform throughout the docs
kill bottom margin on last docs section
Merge pull request #12281 from XhmikosR/vendor

Move docs third-party libraries to a separate folder.
remove border radius when it's not both ends in panel
Fix holder.js links in examples

Replaces links to holder.js with links to the new docs.min.js.
The uncompressed holder.js file is now in docs/assets/js/vendor,
which is excluded from Jekyll builds. In turn, Holder is now
included in docs.min.js, which is smaller than holder.js,
so this should make everybody happy. :)
move migration docs to new separate page, leave callout on getting started page; fixes #12244
Merge branch 'master' of github.com:twbs/bootstrap
Merge branch 'master' into text-emphasis-variant
Merge branch 'master' into bg-variant
Merge branch 'master' into modal-md
Merge pull request #12247 from twbs/text-emphasis-variant

refactoring: add & use .text-emphasis-variant() mixin