Update How-to Guides (#417)

- reduce the number of API keys needed (Use simple tool)
- make everything "tool use" oriented rather than split across agent executor, function calling, tool use, etc.
- Reorg navbar and index
- Fixup some docstrings
- Add more links to ref docs
- Mix up models used
- Simplify a few examples

examples/chat_agent_executor_with_function_calling/base.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Chat Agent Executor\n",
     "\n",
-    "In this example we will build a chat executor that uses function calling from scratch."
+    "In this example we will build a ReAct Agent that uses function calling from scratch."
    ]
   },
   {
@@ -175,7 +175,7 @@
    "source": [
     "## Define the agent state\n",
     "\n",
-    "The main type of graph in `langgraph` is the `StatefulGraph`.\n",
+    "The main type of graph in `langgraph` is the [StateGraph](https://langchain-ai.github.io/langgraph/reference/graphs/#langgraph.graph.StateGraph).\n",
     "This graph is parameterized by a state object that it passes around to each node.\n",
     "Each node then returns operations to update that state.\n",
     "These operations can either SET specific attributes on the state (e.g. overwrite the existing values) or ADD to the existing attribute.\n",