Doing so will allow us to build up releases of the docs as we create
and push to release branches.

In the future we'll have some UI on the website to switch between
versions, but for now you can simple just change 'develop' to
'v1.2' in the URL.

We were checking a version number derived from the branch for whether it
included "rc" - which didn't make sense, branches never have "rc" in the
name. Instead, we use Synapse's __version__ string to perform the rc
check.

There's some potential to using tags instead here, but this works for
now...