[Enhancement/tutorial] Custom constraint tutorial with the Hobby Eberly Telescope

Hi astroplan team!
Thanks for making a great software. I made a tutorial about how to use astroplan with custom constraints for the Hobby Eberly Telescope's fixed elevation design. Is this something that would interest astroplan? I am happy to submit it as a pull request, maybe with a bit of guidance for converting to rst. Here is a link to a gist of the Jupyter Notebook:

https://gist.github.com/gully/c7afb57c805bbd3d87a91d1fdf53791c#file-het_observability_constraints_with_astroplan-ipynb

Start by reviewing the linked HET custom-constraints Jupyter Notebook and the astroplan documentation contribution guidance. Determine how the tutorial should be converted to reStructuredText and where it belongs; done means the tutorial is prepared as a documentation contribution and its custom-constraint example remains usable.

Written by the indexing model from the issue text.