doc/cmd: refer people to cue help for now

Until such time as we solve #9, maintaining a cmd reference by hand is
bound to go out of sync. Give, per #9, we do plan to generate such a
combined reference one day, leave this file in place with a simple
"redirect" asking people to refer to cue help instead.

Change-Id: Ia232437996c27af5a0ed2ad459bfdaeb50d4c27e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9301
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>