Add Astropy Workshop materials to Learn Astropy website

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The Astropy Workshop sequence of Jupyter notebook lessons have been highly successful. Participants have expressed a desire to use the notebooks outside of the workshop. The public could benefit in general from greater exposure to these materials, so we would like to incorporate them into the Learn Astropy ecosystem.

My preferred method is to create a Jupyter book out of the workshop materials.

See the Astropy Workshop pre- and post-workshop summary for more information.