bug#58161: 29.0.50; Update gnus info nodes to explain current versioning

[Stefan Kangas]

Stefan Kangas <stefankangas@gmail.com> writes:

> These info nodes should probably be updated to reflect that development
> is now taking place in the Emacs git repository:
>
>     (info "(gnus) Gnus Versions")
>     (info "(gnus) Ma Gnus")

So I took a stab at it (commit 31302e9cc1).  Feel free to adjust
further.

> While we're here, should the Gnus version be bumped from 5.13 at some
> point?

No comments regarding this, so I guess we don't care.  Closing.