Add gap junction and cell documentation (#706)

Add gap junctions and improve general model overview in docs.

* Replace Arbor Model::Common Types with Arbor Model::Concepts, that introduces the concept of cells, connections and gap junctions. 
* Add gap junction documentation to cpp:recipe
* Document the implicit rule forcing cells connected by gap junctions to be in the same cell group.
* Add documentation that discusses core concepts in Arbor models.