Add documentation chapter about checksums
Data checksums did not have a longer discussion in the docs,
this adds a short section with an overview.
Extracted from the larger patch for on-line enabling of checksums, which
has many more authors and reviewers.
Author: Daniel Gustafsson
Reviewed-By: Magnus Hagander, Michael Banck (and others through the big patch)
Discussion: https://postgr.es/m/
5ff49fa4.
1c69fb81.658f3.04ac@mx.google.com