Move parallel section to decom and replace mentions of smp

[Carldkennedy]

Collaboration needed

Moves current paralell section to decom. Removes mentions of SMP and links. New parallel section, currently includes OpenMPI and GPU (is there a more suitable location?)

.. parallel_bits: comments out sections which may need a revisit once parallel section completed. (Tried to leave ShARC docs as is. Unless we think they need similar editing to avoid confusion. i.e referring to SMP)

Could merge as is, if everyone is happy. Then flesh out parallel section in another PR. Parallel section to include: multi-threading, multi-core processing, Multi-processor, shared memory parallel processing (OpenMP), distributed memory parallel processing (OpenMPI).