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