mpi3mr: Add bare-bones manual

The mpi3mr driver is written by Broadcom for the 9600 Series 24G PCIe
4.0 Tri-Mode RAID Adapters and eHBAs boards. This is 3rd-Generation
Tri-Mode x8 and x16 NVMe/SAS/SATA (although Broadcom/Avago did muddy the
waters by having multiple 2nd generations cards and referring to them
inconsistently).

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