docs: document multiple disks hardware requirement

Miroslav Vadkertirequested to merge
docs/multiple-disks into main

Document requesting multiple disks with different properties via CLI (disk[N].size, disk[N].model-name), API, and tmt plan. Include supported per-disk properties (size, model-name) and infrastructure support notes for beaker, aws, and openstack pools.

Assisted-by: Claude Code Signed-off-by: Miroslav Vadkerti mvadkert@redhat.com

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