Deploy a documentation version for each new Synapse release (#10198)

This PR will run a new "Deploy release-specific documentation" job whenever a push to a branch name matching `release-v*` occurs. Doing so will create/add to a folder named `vX.Y` on the `gh-pages` branch. Doing so will allow us to build up `major.minor` releases of the docs as we release Synapse.

This is especially useful for having a mechanism for keeping around documentation of old/removed features (for those running older versions of Synapse), without needing to clutter the latest copy of the docs.

After a [discussion](https://matrix.to/#/!XaqDhxuTIlvldquJaV:matrix.org/$rKmkBmQle8OwTlGcoyu0BkcWXdnHW3_oap8BMgclwIY?via=matrix.org&via=vector.modular.im&via=envs.net) in #synapse-dev, we wanted to use tags to trigger the documentation deployments, which I agreed with. However, I soon realised that the bash-foo required to turn a tag of `v1.2.3rc1` into `1.2` was a lot more complex than the branch's `release-v1.2`. So, I've gone with the latter for simplicity.

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