Add an example page to the docs on geospatial filtering/indexing

[shoyer]

We cover standard time-series stuff pretty well in the "Toy weather data" example, but geospatial filtering/indexing questions come up all the time aren't well covered.

Topics could include:

• How to filter out a region of interest (sel() with slice and where(..., drop=True))
• How to align two gridded datasets in space.
• How to sample a gridded dataset at a list of station locations
• How to resample a dataset to a new resolution (possibly referencing xESMF)

Not all of these are as smooth as they could be, but hopefully that will clearly point to where we have room for improvement in our APIs :).