ci: create list of supported VMs used in the CI

### Request

Inspect the Buildkite pipelines in the folder `.buildkite` and list what VM images are used as Buildkite runners. Then categorise them by pipeline and type of pipeline step. No need to say anything about the machine type or the disk size.

Create a markdown table that contains:
- the type of Operating System
- the link to the line where it's configured, if possible
- type of step (build, package, unit test, integration test, and so on)
- type of Cloud provider

You don't need to modify anything in the repository but create the file as a comment to this GitHub issue.

### Other details

- Document how you create the plan and how you take action to solve this request