OMG! I really feel like a complete idiot 
But thanks for relieving me anyway 
I created the wiki indeed by creating a page named Home. But I changed my mind and deleted it after the first push… if I remember correctly. What I don’t understand is that these two pages (Core and Overview) were still visible in the side bar until yesterday.
What made me believe they were gone is that I added a link to one of them from my project’s README (as [some text](wiki/Core.md) and the link directed me to the readme again. Then I checked the wiki and freaked out because there was no side bar anymore.
Can that requirement be changed in my project’s settings?
I understand. I’m only doing this because the project is still in its infancy and because I’m the only one in the team. I’ll put the project back to private to avoid people “accidentally” cloning it. So far I’ve been testing Gitlab to make sure I get accustomed to it.