Sidenotes instead of alerts

* I propose: make the docs rely more heavily on the new design language,
replace alerts (not the .alert examples) by sidenotes.
* Remove alert-heading class, which no longer exists in CSS.
Merge branch '3.0.0-docs1' of https://github.com/juthilo/bootstrap-wip into juthilo-3.0.0-docs1
swap all bold p tags in sidenotes for h4s