docs(runtimes): list known external compute drivers #4640

Proposal

The runtimes documentation lists only the built-in compute drivers. Maintainers asked for an Apple container driver to live outside the tree (#1887), and one now exists and passes the conformance suite on v0.1.4. A short "External drivers" section linking known out-of-tree drivers would help users find them and would make the external-driver path visible.