bug#50880: Add "Contributing to Emacs" help command (see C-h C-h)

[Lars Ingebrigtsen]

Stefan Kangas <stefan@marxist.se> writes:

> So how about the attached?  I put it on `C-h C-x' because I figured that
> we might want to use "b" for "bugs" in the future perhaps, and "u" just
> wasn't that nice.  Most of the other ones are taken except for "i", "q",
> "r", "y" and "z".

Looks good to me, but I think this should go into master after the
emacs-28 branch has been cut.

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