doc: update the cephadm download instructions

doc: update the cephadm download instructions

Starting with reef, cephadm is a compiled (zipapp) python application.
The cephadm script has been renamed and thus the old curl-based
download instructions will no loner work. While cephadm still has
no dependencies outside the Python stdlib, this will be changed in
future versions so it is no longer appropriate to just download the
source file of cephadm and run it either.

This change updates the `Install cephadm` section of the doc to explain
how to acquire a "compiled" version of cephadm as well as:
* moving and tweaking the note that the two installation methods are
  distinct
* adding a new note linking to instructions on building cephadm
* moving the distribution-specific installations before the curl-based
  installation to subtly hint that we prefer you to get it using
  packages if you can
* Noting cephadm's minimal required python verision and how to run it
  with a particular python version.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit d11cf0e82aab8d4cef9d423e5d463a373eaf383a)

doc: make instructions to get an updated cephadm common

As discussed in person and over the ceph orch weekly, we want all users
to use a recent supported version of cephadm. Previously, the
instructions only had those downloading cephadm with curl using the
"add-repo" and "install" commands to get a up-to-date cephadm build.
According to ADK we've seen cases of users get "old" distro packages
in the past. Change the instructions so that the "update cephadm" steps
are common after acquiring a "bootstrap copy" of cephadm.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit d7921e88d69b4bc355da9c0327cc33e59e7d7abb)