More verbose README

[pajod]

(Mainly a question to @benoitc) Would you take a patch that considerably increases the size of primary README file?

I was thinking of adding supported configurations and invocation methods, references to pep-3333, to rfc9110+rfc9112, along with a list/table of references to relevant documentation on which non-specced peculiarities we have (input_terminated, e.g. referencing modwsgi and werkzeug), and adding this massive picture generated on where packaged versions are:

I found the repology overview useful, e.g. it make me notice that we are missing out free regression testing against 3.13 because we "demand" all optional dependencies so it goes FTBFS in Fedora 41, and that we still have 2 setgid-related patches from Debian to integrate. (Speaking of patches: I do not seem to have access to backports authored by RedHat, does anyone else?)

EDIT: WIP: https://github.com/pajod/gunicorn/blob/patch-readme/README.rst