added dolfinx notebook to toc

Author: hherlyng

_toc.yml

@@ -7,4 +7,5 @@ parts:
         sections:
           - file: notebooks/intro-mpi/send-recv
           - file: notebooks/intro-mpi/scatter-gather
-          - file: notebooks/send-vs-send
+          - file: notebooks/send-vs-send
+	  - file: notebooks/dolfinx_MPI_tutorial