Point the install instructions at rodent.software

Point the install instructions at rodent.software

Vole's releases are now published on its own instance, so the guides name
that host instead of the placeholder: the .deb URL in each guide, the curl
in the manual install, and the release page where a reader finds both.

The example instance host stays forge.example.com, and stays distinct from
the host the package comes from. It stands for the reader's machine; calling
it rodent.software would invite someone to deploy their own forge under
someone else's name.

The sample in check-deploy.py moves with the guides, so the rendered user
data it checks carries a URL with the shape a real one has.