Wikis are a lot easier to use than they sound
How pages work
Every wiki starts with a Home page. Usually this would look like a quick intro or a set of links to the other important pages, but the format is freeform. Traditionally, wikis are made so that every page is reachable from the Home page, but that isn't a hard requirement.
Pages can be grouped into folders by adding the folder name as part of the title. For instance, creating this page looks like
which creates a page named "Wikis" in a folder named "Features"
Formatting
GitLab wikis use a special version of markdown.
Bold text looks like **this**
Italics look like either *this* or _this_
You can link from one page to another with [Link Title](page-slug), where the "page-slug" is the "codified" version of the name. Linking to this page looks like [Linking to this page](features/wikis).
A header like the one above is as simple as ## Formatting, where the number of #'s determines the header's weight.
To add an image, just click the 
button on the bottom right, and GitLab will automatically add  to show it inline.