There's a bunch of different things all playing against each other here, so let me untangle the low hanging fruit first.
You're unlikely to find a tool like this that isn't paid for simple reason: identity manage and RBAC. In order for it to be collaborative, it needs to have an identity provider as backend: first to auth a user, then to tell them apart (User A updated something User B created...etc). Even if you fork roadmap.sh, I'm pretty sure it's only geared towards a single creator, but maybe I'm wrong on that. Either way, you'll still need an identity provider as a backend if it supports that.
It kind of sounds like a wiki might serve your purposes? You could get creative with page linking and such to make it more interactive, though it may become tedious. Maybe have a look at The Odin Project, which is also OSS, and may have more of what you want to build off of.