We can define regions in our examples that can be referenced
and rendered in guides as code snippets. It is quite hard to ensure
that these regions are maintained correctly. One reason for this is
it is hard to know whether a region is being used or not.
This commit adds a new processor that checks for unused named
regions in examples and fails if any are found.
Fixes#19761
PR Close#40479
Pete Bacon Darwin
George Kalpakas