doc: fixes for cephadm documentation

doc: fixes for cephadm documentation

Be sure to note that python 3 is a prerequisite. Minimal centos 8
installs don't have it, for instance.

Also, we probably don't want to hardcode an octopus URL into the
suggested curl command. Change it to fill that in with
"|stable-release|", which should always point to the latest released
version name.

Fixes: https://tracker.ceph.com/issues/49806
Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit bf69cdc68970789a7410928bd8a1af34d0d9b6a2)