doc: add mlx5 design details
doc: add mlx5 design details

Some details about libibverbs were missing:
- automatic detection by meson
- main ways to access the device

Signed-off-by: Thomas Monjalon <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>
Acked-by: Raslan Darawsheh <18fd192a829ab99382a9c480ddb6085edf3e8489@mellanox.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk