Runner Docs are unclear if GitLab Runner can run without a GitLab installation (aka standalone)

Hi there,

I've been trying to follow the documentation for GitLab Runners as to if I can deploy a Runner Manager which can then deploy individual runner jobs, e.g. via ECS or EC2 in my case. The best I can find is a discussion of "local" usage (like https://docs.gitlab.com/runner/install/linux-manually.html or https://docs.gitlab.com/runner/commands/) but this doesn't answer if it is mandatory to register these runners against an active GitLab instance (self-managed or not). I'm thinking of using GitLab Runners in place of CodeBuild in my AWS CodePipeline setup, but I'm not ready to move my SCM to GitLab yet.

Is this possible/feasible? And if so, would it be worth writing some documentation about this to facilitate something I can imagine being a relatively important use case to people who can't migrate everything to GitLab yet.