epub and pdf downloads of the docs are no longer available

[ftokarev]

Starting from release-2.2 it is no longer possible to download the documentation as epub or pdf files:

release-2.1:

release-2.2 and newer:

There is a discussion about making pdf and epub op-in (https://github.com/readthedocs/readthedocs.org/issues/8359) but it looks like it hasn't been decided yet, so I'm not sure whether it is the cause of this issue.