Docs: add MPI docs
Docs: add local numbering section to MPI
Docs: add comp/comm overlap section to MPI
Docs: add halo exchange section to MPI
Docs: add diagram of decomposed MPI mesh
Docs: add section on MPI distributed assembly
Docs: mark sections in  MPI distributed mesh diagram
Docs: include MPI mesh diagram
Docs: Rewrite MPI local numbering section for clarity
Docs: Rewrite MPI comp/comm overlap section for clarity
Merge pull request #332 from OP2/doc/mpi

MPI documentation