Add reference to Azavea Summer of Maps program

Add reference to Azavea Summer of Maps program like:

> Are you studying geospatial data analysis and cartography? Check out [Azavea Summer of Maps](https://summerofmaps.com/) for details about our other fellowship program.

Start by locating the fellowship-program website content that lists related programs. Use the issue's proposed wording and Summer of Maps link as the acceptance criteria; the work is done when the reference appears on the published site.

Written by the indexing model from the issue text.