docs: Add FAQ item on Jupyter Notebook

Add FAQ answer about Jupyter Notebook.

Closes #217

doc/faq.rst

@@ -0,0 +1,12 @@
+============================
+ Frequently Asked Questions
+============================
+
+How can I use PyStan with Jupyter Notebook?
+  Use `nest-asyncio <https://pypi.org/project/nest-asyncio/>`_. This package is needed
+  because Jupter Notebook blocks the use of certain `asyncio
+  <https://docs.python.org/3/library/asyncio.html>`_ functions. (To verify this, try
+  running ``asyncio.run(asyncio.sleep(1))`` in a notebook.) If you would like to learn
+  more about the problem, see the following issue: `ipython/ipykernel#548
+  <https://github.com/ipython/ipykernel/issues/548>`_. This problem only affects Jupyter
+  Notebook. It does not affect IPython.

doc/index.rst

@@ -81,3 +81,4 @@ Documentation
    reference
    plugins
    contributing
+   faq