Staging view of docs from any branch

[alaric-dotmesh]

Currently, we have a single staging deployment of the docs at https://stage.docs.dotmesh.com/ which tracks the master branch.

It would be nice if it worked more like the dotmesh repo, where builds of any branch get uploaded to https://get.dotmesh.io/unstable/BRANCH/.

So, we could have https://BRANCH.stage.docs.dotmesh.com/ for the docs, auto-built by CI.

To do this, we'd need something like wildcard DNS for *.stage.docs.dotmesh.com routed to an HTTP server with a dynamic vhost mapping, mapping BRANCH.stage.docs.dotmesh.com to $DOCROOT/BRANCH/; and the CI process could then scp generated docs to $SERVER:$DOCROOT/BRANCH.