We tried using the Wiki of a gitlab project in my company to gather some knowledge in a single place, but it was too "hard" to go to a page and edit it. So we moved the content in the repo. Again, we had to search if our content was already written or not, and we were spending time trying to find files and words instead of writing shorts texts.
So we moved our content on the gitlab issues of the repo.
Issues have the same markdown support, we can use ctrl+v to insert screenshots, we can add tags to the issues, we can assign people to relevant issues, we can comment with new things to do/add, and we can use the search bar to find relevant content. All our "knowledge doc" is stored insides the issues of an empty project named "doc".
We were just thinking about something to replace an older system not used anymore, and since it's been 2 years I think I can say that issues really work well for us (small team, 6 devs, some of us write more issues than others).
For personal notes I did what every dev do when they think about a problem ("Why not make my ideal tool myself?"), and I created n.py, a very small Python program that's used to take incremental notes using any program (I used sublime text, nano and now I'm learning vim through it), search in the notes content and display them (repo here: https://git.bitmycode.com/sodimel/n).