Need "images and volumes" documentation

[OpenStack Infra]

This is prompted by https://review.opendev.org/671503 (released in Train; see commit message below).

It's currently documented in the image schema:
https://review.opendev.org/#/c/671503/5/etc/schema-image.json

but it would be good to have some more info somewhere. A likely place is
https://opendev.org/openstack/glance/src/branch/master/doc/source/admin/troubleshooting.rst

The current title is "Images and instances". Maybe change to "Images, instances, and volumes".

There's a section "Instance Launch" that talks about images and volumes, but only talks about volumes as persistent storage. Add a section after it titled "Images and volumes", and mention:
1 - creating a bootable volume (should be something in nova and cinder docs about this that you can copy)
2 - uploading a volume as an image (find something in cinder docs)
3 - uploading an encrypted volume as an image (find something in cinder docs)

The below commit impacts item 3. The release note for it has additional information you can copy:
https://opendev.org/openstack/glance/src/branch/master/releasenotes/notes/bp-barbican-secret-deletion-support-40cffa5ffa33447e.yaml

https://review.opendev.org/671503
commit b190a39a2831d4b221476444c09d5f1bd88f6fd2
Author: Cyril Roelandt <email address hidden>
Date: Wed Jul 17 22:15:10 2019 +0200

    Delete secret key on image deletion

    We add two extra properties for images:
    - cinder_encryption_key_id, which stores the encryption key id;
    - cinder_encryption_key_deletion_policy, which states whether the secret
      key should be deleted on image deletion.

    This feature uses the Castellan key manager, and will therefore work
    with all its supported backends.

    Implements: blueprint barbican-secret-deletion-support
    DocImpact

    Change-Id: Iacd0b3785ad4cdd06961e6d11967775806e009ff