Document internal Executor Interface
What does this MR do?
Adds documentation about the internal Executor Interface and it's usage by Docker Machine Executor's autoscaling provider.
Why was this MR needed?
As part of Autoscaling Provider for GitLab Runner to repla... (&2502) we've recently went through an architectural blueprint exercise. The current state of the design proposal can be seen at https://docs.gitlab.com/ee/architecture/blueprints/runner_scaling/index.html.
One of the next steps we've decided to do is to document an internal executor interface that Runner is using and point how Docker Machine Executor is making use of that interface to provide autoscaled fleet of VMs for Docker Executor usage. This will be our entrypoint to discuss how we should expose the interface to introduce a new autoscaling solution as described in the blueprint.
This MR is created to provide such documentation.