There's a couple of other types in the public interface of Tree that should
be teased out, and it feels odd to have one type in one file and a bunch of
others in another. So this sets us up for having them all the same place.
This should make it easier for the reader to find related information.
While it's arguably not the most useful widget, it's in the code, exported,
and can be seen by people; I feel it should be in the documentation. I've
ensured that it also links to Placeholder given it sort of is a Placeholder
and so people might arrive at it first and it makes sense to redirect them
to something more comprehensive.
A large part of the code to go with this is to show off a radio set; I feel
it makes sense to use the same code for both bits of documentation given
that a radio button only really makes sense inside a radio set.
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.
