Add config option to warn on missing example payloads

[jautor]

As response payload examples (and for action responses) are picked up automatically from a folder when specified, it would be helpful to throw a warning when an example is NOT found - since the intent of the output is to fully document the schemas. But making it an option to not annoy other users makes sense...

[akf]

Which is the default, "nag" or "don't nag"?

[jautor]

The universal default should always be "don't nag"... 😄

(I will turn the option on as I'll use the feature, but don't expect all use cases would want that)

[akf]

"warn_missing_payloads"