docs: add app to events usage example

[juan-riveros]

Pull Request Checklist

• New code has 100% test coverage
• (If applicable) The prose documentation has been updated to reflect the changes introduced by this PR
• (If applicable) The reference documentation has been updated to reflect the changes introduced by this PR

Description

• Adds the app definition so that the example is provides all the information needed to make a functional example.
• Currently, users will receive a ImproperlyConfiguredException(f"no event listeners are registered for event ID: {event_id}") as they aren't made aware to register listeners or where to register them.
• Users would have to dive into the code to determine where the listener should be registered, this change exposes the need to add listeners to the application.

Feel free to disregard this if I am just misusing this or approached it incorrectly.

[cofin]

@all-contributors add @juan-riveros for docs

[allcontributors]

@cofin

I've put up a pull request to add @juan-riveros! 🎉

[cofin]

Thanks for taking the time to correct this! I'll merge as soon as it's validated.

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/2191