Any of the above will work as long as everything’s well organized and easy to find. Personally I find Google Docs and Confluence impossible to locate stuff in.

Any of the above will work as long as everything’s well organized and easy to find. Personally I find Google Docs and Confluence impossible to locate stuff in. Markdown docs within your main repository (or a central docs repository if for some reason you have tons of repos, though IMO that’s an antipattern for a small team) should work fine at least for a team or up to 20 or so.