It would be nice to keep RT-related things together under the RT umbrella which, for better or for worse, is currently on GitHub. But since you’re running the documentation server (and maybe at some point the website itself will migrate to your server), would hosting things on our GitHub soon-to-be organization complicate documentation deployment for you @patdavid ? It makes just about no difference to me whether the documentation uses GitLab or GitHub. If it complicates things for you then I have nothing against using GitLab.
The nice thing about git is its distributed nature, so if GitHub goes rogue we can pop over to GitLab or whatever the future holds with a hypothetical click.
On a related note, accepting documentation changes through PRs will improve the documentation, since someone from the core team will have to approve the changes and merge the PR. I’ve had bad experience with MediaWiki’s model, where contributors once approved can edit anything, so someone approved just to translate would go on and make changes to the source English text, and so I’d find that someone wrote pure nonsense a year or two after said nonsense was published.