Show two larger application examples. One example that
could be used in a CLI, the other in a long running
service. These demonstarates different strategies for
handling DB.Ping errors in context.

Fixes #23738

Change-Id: Id01213caf1f47917239a7506b01d30e37db74d31
Reviewed-on: https://go-review.googlesource.com/c/101216Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>