I have used a fair number of systems at this point, all of which seem to have pros and cons.
For collaborative editing - multiple users editing at once - web-based tools are going to win this. Google Docs is the clear winner for academia, because the alternatives don't have as robust a set of tools for reference management. If you only need document editing, it looks like cryptpad is a promising option not yet discussed. You can stand up an instance inside the walls of your organization, or even run a public-facing instance with less probable exposure because of the encryption built in. Also, your organization's data isn't being used to feed LLMs by Microsoft or Google, which is a nice change.
Confluence, already mentioned, is not good at collaborative editing, even though it is a wiki. The only reason anyone has Confluence (I believe) is because they bought Jira. I imagine that this can unify your whole workforce in hating Atlassian, instead of hating management.
I have used whatever Microsoft is calling its document sharing platform - SharePoint, Teams, etc - and it is fine. OneNote is actually capable of collaborative editing, which is kind of OK. It all gets sucked into the LLM machine, and Microsoft will never miss a chance to charge you rent on things you thought you owned, but it mostly works.
I have used Dokuwiki, mentioned below, and it is surprisingly capable, though I would not try anything too complicated, there is little conflict management built in.
While I like Markdown, and using a source code management tool is very robust, I would only try that with other programmers - most people who are not programmers are profoundly insensitive to syntax, and even very competent Excel jockeys don't think of what comes after the "=" to be syntax. For markdown to work well, people have to be able to express themselves by knowing the syntax, and that is a non-starter in my experience.