The idea here is that there is a single file for the nav for all the docs,
both online and offline. The nav will be the full online nav in all its blog
glory, and then the build system will strip out the nav items relating to
the blog when building the offline version.
There's a wrinkle or two still in here; the main one being that when doing a
docs-serve, it seems that the cleaning up of the online nav doesn't happen
when you Ctrl-C out of the server. I'm not 100% sure of the cause of
this (there could be a couple of different reasons).
For now... I'm ensuring that the online/offline nav files are excluded from
git. It would be nice to find out how best to always ensure that the online
nav file is removed so nobody attempting to update the docs gets confused
about what to edit.
This will build the docs into a directory called docs-local, sans the blog.
Note the near-total copy/paste of the `nav` due to how mydocs does
configuration inheritance; this is fine for now.
