doc: add shared guide for mlx5 drivers

doc: add shared guide for mlx5 drivers

Adds new documentation for MLX5 common driver that contains:
 - Its features list (doesn't exist for now).
 - Its devargs description.
 - Device configuration information and tutorial.
 - Quick Start Guide for Mellanox OFED/EN.

Move into this doc all shared information from other MLX5 PMD docs and
add them reference to new common doc.

Signed-off-by: Michael Baum <michaelba@nvidia.com>
Reviewed-by: Raslan Darawsheh <rasland@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>