I don’t think a built-in wiki should be a priority for Lemmy. The sysadmin of an instance.com instance can host a separate web app as a standalone wiki at wiki.instance.com.
For example, you could host an mdbook at this subdomain to serve as a docs-style wiki.
A lot of games on Steam are DRM-free, but not (yet) on GOG. GOG isn’t an afterthought just because of their DRM-free policy, it’s also because they’re so small.