Add a section to the upgrade doc instructing users to (and how to)
migrate `CephRBDMirror` `peers` spec to individual `CephBlockPools`.
Adjust the pending release notes to refer to the upgrade section now,
and clean up a few references in related docs to make sure users don't
miss important documentation.

Signed-off-by: Blaine Gardner <blaine.gardner@redhat.com>

Remove legacy documentation for configuring RBD mirroring. While we
still support legacy mirroring configs, we want to encourage new users
to use the CephBlockPool configuration for mirroring.

Signed-off-by: Blaine Gardner <blaine.gardner@redhat.com>