This commit adds a bit of helper code to the top-level package.json to
always ensure that the `themes/docsy` submodule is checked out and that
its nodejs dependencies are installed. Thus, we can remove the need to
download git submodules directly. Instead they will be loaded when running
`npm i`.
We also don't need to load submodules recursively anymore, as docsy has
removed any submodules from its repo since the last time we updated the
theme.
The `--depth 1` bit of `git submodule update --init --depth 1` tells git
to only check out the latest commit of any submodules - saving bandwidth
and space.
