Enable readthedocs offline formats

Hello, I'd like to ask for enabling the offline formats in the readthedocs config. This will let us download PDF files per version.

[Offline formats](https://docs.readthedocs.io/en/stable/downloadable-documentation.html)
[Enable offline formats](https://docs.readthedocs.io/en/stable/guides/enable-offline-formats.html)
[More on StackOverflow](https://stackoverflow.com/a/52131151)