Add cmd section to experimental

[jbergstroem]

References this community post: https://community.fly.io/t/change-a-containers-cmd-via-the-fly-toml-file/2536

[jsierles]

We have another entrypoint which is for changing the equivalent of the Docker ENTRYPOINT as opposed to the Docker CMD. However, this is a good starting point. So I will merge and clarify the two options and add docs for the multiprocess preview mentioned in that post.

Thanks!