Add doctest for add_vertex in GraphAdjacencyList. Contributes to #9943 (#13143)

* Add doctest for add_vertex in GraphAdjacencyList. Contributes to #9943

* Update graph_adjacency_list.py

---------

Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

graphs/graph_adjacency_list.py

@@ -61,6 +61,15 @@ class GraphAdjacencyList[T]:
         """
         Adds a vertex to the graph. If the given vertex already exists,
         a ValueError will be thrown.
+
+        >>> g = GraphAdjacencyList(vertices=[], edges=[], directed=False)
+        >>> g.add_vertex("A")
+        >>> g.adj_list
+        {'A': []}
+        >>> g.add_vertex("A")
+        Traceback (most recent call last):
+        ...
+        ValueError: Incorrect input: A is already in the graph.
         """
         if self.contains_vertex(vertex):
             msg = f"Incorrect input: {vertex} is already in the graph."