docs: Tidy up runtime documentation.

- Drop support for podman since it is untested. It's simple to add it
  back in future if requried.
- Make `docker` the default runtime; this is what we want to push users
  towards.
- Improve runtimes documentation to include discussion of the image
  variants and dependency requirements.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>