Move container image documentation to a separate section

### Follow-up of https://github.com/iterative/cml.dev/issues/86#issuecomment-900989038 and https://github.com/iterative/cml.dev/pull/99#discussion_r702835302

I wonder if we should extract the container image documentation to a separate section, instead of keeping it inside the self-hosted runners section. Images are being used in many other scenarios, like running any CML command on CI/CD systems other than GitHub Actions.