doc/cephadm: explain different methods of cephadm delivery

doc/cephadm: explain different methods of cephadm delivery

Explain that only in Reef and later releases is cephadm distributed as
an executable compiled from source code. This note is to go into Quincy
and only into Quincy, to direct new users of Ceph whom circumstance has
delivered into the hands of Quincy and who might have the wrong idea
that the documentation of Reef and later releases applies to their
release.

Signed-off-by: Zac Dover <zac.dover@proton.me>