devlink: rename and expand devlink-trap-netdevsim.rst

Rename the trap-specific netdevimsim.rst file, and expand it to include
documentation of all the devlink features currently implemented by the
netdevsim driver code.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>