Closes #2094

Update the ImageOverlay example in the docs that shows how to use a local file. The previous example file is no longer included after the docs overhaul. This example breaking was not spotted because the code there is robust to the file not existing.

• Use another image that exists in the docs static files: the Folium logo. Easy and for the example it doesn't really matter I think.
• Make it raise an error if the file not exists. That way we will be notified if this example breaks in the future.

Screenshot