If the central use case are .md files and common language, I’d still think Discourse has a big advantage in that it is conversational and supports people actually talking to each other in plain language. As you said, you generally want that conversation about knowledge happening on Discourse. So I think it’s worth looking at how to keep most of it right there, otherwise you drive attention and focus away from what you’d want as your main platform.
I could imagine a setup like:
- Readme.md as a wiki topic on Discourse, synced to GitHub
- Discussion about this Readme as follow-up-posts on Discourse, regularly deleted (similar to the current setup on meta)
- Additional files and folders in a repo handled on GitHub and with branching. The current repo structure could be synced to Discourse, so that it shows up with the wiki post like a table of contents.