Add section about differences between GitHub and GitLab

[lnnrtwttkhn]

From my experience, there are no major differences regarding basic Git operations (push, pull, clone). (Some) minor differences I noticed: UI looks different (obviously), pull requests are called merge requests (which makes more sense IMHO), a bit more fine-grained accessibility levels, CI/CD works differently, GitLab Pages of private repos require login (cool feature IMO).

Some additional resources:

• [ ] https://vimeo.com/668637653
• [ ] https://codebase.helmholtz.cloud/hifis/software/education/hifis-workshops/introduction-to-git-and-gitlab/workshop-material
• [ ] https://codebase.helmholtz.cloud/fritjoflammers/docker-training/-/tree/main/setup-git-gitlab?ref_type=heads
• [ ] https://gitlab.com/HeidiSeibold/setup-git-rstudio-gitlab