Document Repo /docs/:package-:version.tar.gz endpoint

[wojtekmach]

This would be an optional endpoint that a repository might implement. It would be used by e.g. mix hex.docs fetch foo 1.0.0 --repo acme to download the docs tarball. (Note, we don't yet have --repo on mix hex.docs.)

Another option would be for mix hex.docs fetch --repo to instead use the HTTP API /packages/:name/releases/:version/docs endpoint for custom repos but that doesn't seem ideal either.