doc: Remove empty README.md and relocate nvmeof HA design doc

doc: Remove empty README.md and relocate nvmeof HA design doc

doc/README.md was a 0-byte file with no content; remove it.

doc/nvmeof/ha.md described the internal NVMeofGwMon/NVMeofGwMap
HA implementation (Paxos messaging, ANA state machines, blocklist
logic) -- a developer design document, not user-facing content.
User-facing NVMe-oF documentation already exists in doc/rbd/.
Move to doc/dev/nvmeof-ha-design.md where developer design docs live.

Fixes: https://tracker.ceph.com/issues/77208
Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>