Add section to intro tutorial on graph element modification (#596)

This commit adds a section to the introduction tutorial on graph element
modification. It's a basic feature of interacting with a graph to show
how you can update a node or edge in place on an existing graph object
however this was previously not mentioned in the introduction tutorial.
This commit corrects this oversight by explaining how this can be done.

(cherry picked from commit d02b544)

docs/source/tutorial/introduction.rst

@@ -189,6 +189,30 @@ subsequent additions. For example, building off the previous example if you ran
 
 this new node is assigned index 2 again.
 
+Modifying elements of a graph
+=============================
+
+The graph classes in retworkx also allow for in place mutation of the payloads
+for elements in the graph. For nodes you can simply use the mapping protocol to
+change the payload via it's node index. For example:
+
+.. jupyter-execute::
+
+   last_index = graph.node_indices()[-1]
+   graph[last_index] = "New Payload"
+   print(graph[last_index])
+
+You can update the payload of any node in the graph using this interface. For
+edges you can leverage the :class:`~.PyGraph.update_edge` or
+:class:`~.PyGraph.update_edge_by_index` methods to update an edge's payload
+in place. For example:
+
+.. jupyter-execute::
+
+   edge_index = graph.add_edge(0, 1, None)
+   graph.update_edge_by_index(edge_index, "New Edge Payload")
+   print(graph.get_edge_data_by_index(edge_index))
+
 .. _data_payload:
 
 What to use for node and edge data payload