Add PS1 20 queries spatial binning notebook

[sedonaprice]

As part of the CASJobs sunsetting efforts, MAST will be providing notebooks demonstrating how to query the Pan-STARRS catalogs at MAST using TAP/ADQL as an alternative access method.

While a Pan-STARRS/TAP notebook already exists within the MAST notebooks, the set of Pan-STARRS "20 queries" (based on the SDSS 20 queries presented by Gray et al. 2002) as adapted and formulated by Rick White for CASJobs (available on innerspace) were identified as additional valuable examples.

This notebook presents the "spatial binning" subset of these "20 queries" for Pan-STARRS, covering queries mapping objects in spatial bins (over RA/Dec -- Q12; and in HTM bins -- Q13).

Overview pages for PS1 and the PS1 "20 query" tutorials have been added, following the style of the SDSS notebooks. (These will require merge editing given the other PS1 20 queries pull requests.)

As a note: two things will require editing before merging. Both items are flagged with big red boxes, which will be removed prior to merging:

• Verifying integration of the schema browser (and editing the narrative to describe how to access the schema browser), after it has been deployed publicly (as a URL) and is integrated in TIKE (through the jupyter hub instance).
• Adding correct URLs to the "PS1 TAP migration guide", describing differences in the MSSQL-backed and new, more-performant postgres-backed TAP service for PS1 DR2.

[sedonaprice]

@ttdu Ready for narrative review!