Publish old versions on ReadTheDocs

Right now, https://postgresql-anonymizer.readthedocs.io/en/latest/ only makes the "latest" and "stable" versions of the docs available. It would be nice if the RTD selector in the bottom right also let you pick old versions of the extension. In particular, I was interested in the docs for 0.8.1 because this was the last release that was possible to install on RDS (though I know this is hacky and unsupported). Because this wasn't available, I had to dig around in the git history which isn't as nice as just looking at RTD.

Convenience link to activate branches of old versions: https://readthedocs.org/projects/postgresql-anonymizer/versions/