doc/mgr: edit orchestrator.rst

doc/mgr: edit orchestrator.rst

This PR improves the English language in the "Orchestrator CLI"
section of the MGR documentation. It adds a couple of section
headers in order to signpost the information in the document
a bit more than had already been done, but it makes no major
structural changes to the presentation of the information here.

This PR was motivated by feedback from the 2022 Ceph User Survey
in which one of the respondents wrote "better ceph orch documen-
tation".

The final section on this page, "Current Implementation Status",
must be verified by someone who is familiar with the current state
of "ceph orch" and a date stamp should be applied to the top of
the section so that the word "current" has a meaningful referent.

Signed-off-by: Zac Dover <zac.dover@gmail.com>