The devstack plugin docs mostly referred to in tree plugins, which is
honestly something we don't want people doing. Instead restructure the
whole document to talk about external plugins as the only kinds of
plugins, and focus on a workflow to make that easy for people to work
through.
This also adds a plugin-registry page to start listing known plugins
somewhere centrally. Some sample content was added, hopefully people
will submit patches to include their plugins.
This does drop the section on hypervisor plugins. That's not currently
something that we expect a ton of people to work on, so diving into
the code for this should be fine.
Change-Id: Ifc0b831c90a1a45daa507a009d1dcffcd6e2deca
0124e08ede