A new chapter for Hash Indexes, designed to help users understand how
they work and when to use them.

Backpatch-through 10 where we have made hash indexes durable.

Author: Simon Riggs
Reviewed-By: Justin Pryzby, Amit Kapila
Discussion: https://postgr.es/m/CANbhV-HRjNPYgHo--P1ewBrFJ-GpZPb9_25P7=Wgu7s7hy_sLQ@mail.gmail.com