Rename all the documentation files to use dashes in URLs rather
than underscores, matching our other documentation sites. Hopefully
this is early enough in the life of the mobu documentation that
there aren't many external links. I'll fix any that crop up in
Phalanx link checks.

In the GitHub Actions configuration, remove the cache key prefix
for the tox step in the docs build. There should be no reason why
we can't use the same tox cache as is used for the regular checks.