Add model examples

It is very useful for developers to be able to visualize how a specific model, request body or response body might look instead of just seeing the attribute type.

There are several ways to do this, which might look differently depending on which HTML template we are using (see #182):

• setting schema.example for each model attribute. This approach is probably the best. I think some HTML template might require this to be called x-example instead.
• setting response.examples for each endpoint response.

Review issue #182 and the existing model and endpoint response generation entry points. Compare the schema.example and response.examples OpenAPI options described here, including how the selected HTML template handles them. Done means the chosen approach produces visible examples for model attributes and request or response bodies.