Document a way to produce a geo-restricted set of taxa for TOL subsetting or custom label predictions

[thompsonmj]

Users have expressed interest in generating lists of taxa restricted to a certain region for use with pybioclip for use with --subset, --cls, and/or --bin features.

GBIF's web UI is the simplest approach to generating this list, so adding a tutorial on how to do this to the documentation would be beneficial for these users.