Bug#971623: link to wnpp.d.net

Hi,

I just looked at the wnpp.d.net page and found it very nice. Looking
at the git logs of the repo for this domain, you can see that it's
very well maintained and in 2021 the old php code was replace with
django and Python 3. So, this is better maintained than some very old
code in our webwml repository.

I find the idea of linking to wnpp.d.net also very valuable.
And maybe wnpp.d.net will become a debian.org domain in the future.