Build release specific RTD docs, link to current release when not deploying dev branch

[mikkonie]

Currently, the logic to link out to RTD in SODAR is as follows:

• If we are running SODAR on a development commit, link to dev
• Else, link to latest

However, this may not always be what we want. I can think of multiple cases where this may fail, some already seen in real life:

• If an instance stays at an older release version, purposefully or otherwise, users are directed to documentation for a newer release which may be vastly incompatible
• Tagging a release when deploying an older non-dev commit creates the same situation

This should be fixed as follows:

• Build docs by version number for each release
• If on a release commit, link to docs for that release
• Else, link to dev like we do now

We used to build release-specific docs but it seems we don't do that anymore? Maybe I've messed something up in the RTD configuration.