Docs additions

• Include notes on relationship to qFaults.
• Update criteria sections

Other notes from RG:

1. The National Seismic Hazard Model (NSHM) source faults represent an inventory of faults used for hazard modeling purposes. They are a simplified representation of known faults that have been active throughout the Quaternary (e.g., Quaternary fault and fold database, Qfaults).
2. NSHM source faults were identified/selected largely based on frequency of ground-rupturing earthquakes and slip rate (>0.1 mm/yr). The USGS seeks input from the earthquake community (federal, state, and local government, academia, industry) during each update to the NSHM, so they are viewed as a community/consensus product, though we recognize there is always room for improvement. The USGS is currently in the process of reviewing and updating the source fault model that will be used in the 2023 NSHM update.