Documentation at readthedocs.io outdated

[Blackoverflow]

I noticed that the documentation at https://froide.readthedocs.io/en/latest/ is severely (1975 commits right now) outdated.

1. https://froide.readthedocs.io/en/latest/herokudeployment/ is no longer present in main
2. The links "Edit on GitHub" point to the branch "master"

I'm pretty sure readthedocs is still configured to follow "master".

Since this needs to be configured in the settings of the readthedocs account, I can't provide a merge request to fix this.