We started with a wiki (since we were on GitHub, we used the one provided with our main repo). We eventually switched to using a separate repo containing only markdown files named meta for all of our documentation, including research, best practices, corporate how-tos like expense reporting, time off protocols, etc. We even had a page dedicated to the memes we use in chat, so new team members could figure out what the heck was going on.
We found using a markdown repo gave us a few advantages:
You mentioned some non-technical documentation is in the repo too -- how have non-devs managed to deal with that? Do your non-technical teams use a different system for documentation?
Since we were a GitHub shop, we just had the greater product team (product management, customer success, etc) set up GH accounts, so that any URL to documentation was accessible. That at least gave everyone read-only access to the docs. We did end up teaching some of these people enough git-fu so that they could contribute, as part of their professional development, and that was well-received. But I did feel this setup was more dev-centric than I would have liked, and it ended up duplicating some information that was contained elsewhere, like in the employee handbook. But I have a violent allergy to both SharePoint and Confluence so this worked well enough for us.
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