Move Implementations section to a wiki

[strugee]

Or whatever.

https://tantek.github.io/post-type-discovery/index-src.html#implementations

[tantek]

original title suggested W3C wiki, but per "or whatever", likely IndieWeb wiki where existing implementations are documented right now unless there is a good reason to move it

[strugee]

I have literally zero preference; IndieWeb wiki sounds great!