doc/ceph-volume: update stale ceph-disk references

ceph-disk was deprecated in Ceph 13.0.0 and has since been removed, but the
ceph-volume docs still described it as a current, deprecated tool and carried
extensive historical migration narrative.

* index.rst: state that ceph-disk has been removed rather than "is deprecated"
  with warnings that no longer appear; keep the two migration paths for
  clusters that still have ceph-disk OSDs
* intro.rst: collapse the historical "why ceph-disk was replaced" narrative
  into a brief note, preserving the ceph-disk-replaced anchor
* man/8/ceph-volume.rst: describe ceph-disk in the past tense

The ceph-volume simple subcommand still manages legacy ceph-disk OSDs, so its
documentation is left in place.

Fixes: https://tracker.ceph.com/issues/77186
Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>