Add an example about including other assets such as images (png, svg)

[jtpio]

Problem

Some extensions may want to provide and display custom assets, such as png or svg files.

For now there does not seem to be many examples on how to do that in an extension.

Proposed Solution

• Show how to include a png and/or svg, and show it in a main area widget
• Mention the use of a custom webpack.config.js to provide custom rules: https://jupyterlab.readthedocs.io/en/latest/extension/extension_dev.html#custom-webpack-config