ReadTheDocs: document all stable versions since 2020

[tristanlatr]

We started the sphinx based documentation in 2020 but we did not properly configure nor document how to handle different versions of pydoctor. This resulted in the default behavior that being to only document the latest and stable versions.

The issue is that users might have an older version (especially of they pull it with apt-get). So having the correct list of supported options for a specific pydoctor version becomes important.

Additionally, I’m quite frustrated to see the very old (2010) man pages of pydoctor show up in google search result when looking for « pydoctor manpage »:

• https://www.unix.com/man-page/debian/1/pydoctor
• https://manpages.debian.org/unstable/python-pydoctor/pydoctor.1.en.html
• https://manpages.ubuntu.com/manpages/trusty/man1/pydoctor.1.html
• https://man.fyi/ubuntu1210/1+pydoctor
• https://manpages.org/pydoctor
• https://www.venea.net/man/pydoctor(1)

I’de like to have all of them updated.