Final revision to 1887 update developing plugin documentation

This will be the last revision to #1887 (closed) before merging. There are still a number of changes being made based on the discussions in the MR here.

@pizarroj While I am waiting for feedback from Alvin, could you take a look at the computational section and provide a review? (No rush, I am not expecting to hear back from Alvin till at least next week).