collapsible nav section in docs

read-the-docs uses reStructuredText with Sphinx, whereas we're using markdown with mkdocs. Sphinx has more options, but is also harder to configure than mkdocs. Fwiw, we'd started out with rst/sphinx.

See: https://raw.githubusercontent.com/rtfd/readthedocs.org/master/docs/index... for how they use the toctree directive.

I'm all for having better nesting in menus using mkdocs. This was a limitation until recently. See: https://github.com/mkdocs/mkdocs/issues/6.

I believe there was an issue on rtfd to update to a more recent mkdocs to support the extra menu depth. I guess we need to figure out of rtfd has implemented that yet... Looks like they're running a recent mkdocs: https://github.com/rtfd/readthedocs.org/issues/1277

So, we should probably create a branch to play with the new menu structure in https://github.com/aegir-project/documentation/blob/3.x/mkdocs.yml. One reason I like mkdocs is that it's super simple to compile/run your own local copy, to see how everything works.

One possible issue here is that mkdocs had to update their themes to support the additional menu nesting. According to https://github.com/rtfd/readthedocs.org/issues/978, these themes may not yet be supported. This'll also need some investigation.

I added a 'restructure' branch, to see how the new menu/page structure works: http://aegir.readthedocs.org/en/restructure/

I also collapsed the individual hooking-into pages into a single page, which I think we should also consider for remote-import.