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>