devlink: add a devlink-resource.rst documentation file

Take the little bit of documentation for resources from various commit
messages and combine it into a new devlink-resource.rst file.

This could probably be expanded on even further by someone with more
knowledge of how the devlink resources work.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>