Allow packages from sources other than GitHub

Moving over here from https://github.com/purescript/registry/issues/15, which mentions that the only problems of allowing packages to be published from sources other than GitHub come from Pursuit relying on GitHub for some things. Quoting from there:

Right now Pursuit only supports packages hosted on GitHub, and nobody has complained [..]. The reasons for this are that allows us to use the GitHub API to get a rendered HTML readme (although I now consider that a misfeature and I'd like to remove it), and also it allows us to construct source links, since the source links take you to GitHub. If we allow non-GH packages in the registry, this means we won't be able to upload those packages to Pursuit right now (which is not a dealbreaker but probably worth considering). I'd quite like to have package sources hosted in the same place as the HTML API docs for source links though, which will probably be easier to do now with our shiny new registry; if we did that we'd be able to allow non-GH packages without losing source links.

Start by reading the linked PureScript registry issue and tracing how Pursuit currently relies on GitHub for rendered READMEs, package uploads, and source links. Identify the relevant package-source and documentation paths before proposing a design. Done should mean non-GitHub packages can be accepted without losing source links or required Pursuit documentation support.