Add more advanced, more science-focused map_partitions example

Following up on #536, add another example in the same vein, but demonstrating use of 2-D histograms, in an example like this one, https://docs.lsdb.io/en/stable/tutorials/pre_executed/des-gaia.html#Plot-the-HR-diagram, but showing how map_partitions assists.

Start by reading issue #536 and the linked DES-Gaia tutorial section on plotting the HR diagram, then inspect how the existing map_partitions example is organized. Add a science-focused example using 2-D histograms and demonstrating map_partitions; it is done when the example clearly shows that workflow.

Written by the indexing model from the issue text.