[cooker] Forum post Source code hosting tools comparison

As there is no collaborative sheet link shared that I'm aware of, you
will find here a wiki-fied post on the subject:
"wiki-fied" means that the users can edit it like it was a wiki page,
and all revisions are kept in history.


I've adjusted the formatting for readability and done some basic
filling in of the Pagure comparison.

I don't know what the "Distro-independent" property is supposed to
mean. If it means it wasn't created by developers of a distro, I don't
know if that's necessarily a good thing, given that developers of a
distro making Git forge software would be able to better understand
our needs more than anyone else...