More work on the sidenotes

* Convert more inline advices (compatibility, dependencies, ...) to
sidenotes
* Add sidenotes for:
** Note on plugin dependencies in general
** Required padding for navbar-fixed- (incl. code snippet)
* Fixed text for the small tag
More contrast with code snippets in sidenotes
use spaces for indenting, not tabs /cc @juthilo
shorter copy
lighter .highlight in sidenotes as well
more copy edits to sidenotes