doc/start: add testing support information

doc/start: add testing support information

This PR adds information about support for testing,
and information about which distros the Ceph project
builds packages for.

This is one in a series of PRs including the following:

https://github.com/ceph/ceph/pull/45385
https://github.com/ceph/ceph/pull/45764

This PR specifically includes the information that Ernesto
Puerta collected here:
https://github.com/ceph/ceph/pull/45385#pullrequestreview-911766656

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