DoIT Shared Tools - GitLab - GitLab Wiki Feature and Snippets
You may create documentation via the GitLab Wiki feature. Another method of documentation is using GitLab Snippets.
You may have wiki style documentation in your GitLab project. It all depends on exactly what your goals are. Often, the most common thing we hear from other Shared Tools customers is that neither of these features beats the simplicity of putting Markdown files in the repository itself and using those files as your documentation.