Add documentation section about services

doc/services.rst

@@ -0,0 +1,19 @@
+External Services
+=================
+
+Explain the use of services.
+
+Modelverse-side
+---------------
+
+Explain how services work in the Modelverse.
+
+Service-side
+------------
+
+Explain how services work in the Python wrapper.
+
+Example
+-------
+
+Give a full example of a service.