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/high-level-tools.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Chat Executor: with tool calling\n",
     "\n",
-    "This notebook walks through an example creating a chat executor that uses tool calling.\n",
+    "This notebook walks through an example creating a ReAct Agent that uses tool calling.\n",
     "This is useful for getting started quickly.\n",
     "However, it is highly likely you will want to customize the logic - for information on that, check out the other examples in this folder."
    ]