To be more specific about what I was saying earlier: the differences in form factor, features, layout, etc. are all going to be unfamiliar to those who don't know GitLab--so, presenting them a migration guide to GitLab from GitHub that's also already hosted on GitHub, with the layout and all the features they're already familiar with, is going to have a higher conversion factor than otherwise. It's not necessary that they randomly browse to this guide. Guides work because there's a smooth transition between the easy/familiar and the objective. By starting on the familiar, the guide is more inviting to follow.
I mean, it's just a git directory with a README.md--it's not like it can't be both places, but the argument from dogfooding I don't think is a good one.