Add docs for jupyterlite_contents values

Author: jasongrout-db

docs/configuration.rst

@@ -12,6 +12,8 @@ You can embed custom content (notebooks and data files) in your JupyterLite buil
 
     jupyterlite_contents = ["./path/to/my/notebooks/", "my_other_notebook.ipynb"]
 
+``jupyterlite_contents`` can be a string or a list of strings. Each string is expanded using the Python ``glob.glob`` function with its recursive option. See the `glob documentation <https://docs.python.org/3/library/glob.html#glob.glob>`_ and the `wildcard pattern documentation <https://docs.python.org/3/library/fnmatch.html#fnmatch.fnmatch>`_ for more details.
+
 JupyterLite dir
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