With #631 in mind, start by moving the vast bulk of the documentation
configuration into a "common" file, and just keep the `nav` in the main
configuration file. The thinking here is that I want two entry points -- one
for building the full docs for the Textual website, and one for building a
local version of the docs.
Because mkdocs doesn't allow for inheriting a configuration and also
splicing into the `nav`, we'll be looking to duplicate the `nav` for now --
this will likely change eventually as that'll be a maintenance overhead that
we don't really want (will likely look at templating it or something). For
now though, let's look at the best way of splitting things up and seeing how
we can generate two different versions of the docs.
