Thanks for all the replies to this. It's taken me a while to understand the various responses I got to this idea but it's gradually become a bit clearer. It seems that there are actually several possible Pd wikis that could be made or combined:
A wiki of Pd objects - a sort of in-depth illustrated help with tips and examples for every object and external
A wiki of Pd methods and applications - how to use Pd; e.g. handling midi, random functions, FFT, etc
A wiki of Pd resources, links, and examples - a guide to what is available online, Pd tutorials, instruments - a kind of collective annotated bookmarks collection
The last one is what I personally had in mind initially and I have my own version here:
https://docs.google.com/document/d/1tzS2KU8x31JXoUxmkEr5WikJvxcrgHa4C0vM8LBw49Q/edit?usp=sharing
which anyone is welcome to join in on. It's currently a bit of a mess and not sorted into any real system. I understand from certain replies that some people don't think it's a good idea to make access to other people's work too easy but I think the more fun you have with Pd, without having to learn everything the hard way, the better.