Add required PR check ensuring `examples.json` doesn't break mdn/rari

[caugner]

See: #639

This will require upstream work in Rari, as the URL is currently hard-coded:

https://github.com/search?q=repo%3Amdn%2Frari%20examples.json&type=code

[Rob--W]

'ensuring that rari builds" sounds computationally expensive. An alternative without dependency would be a linter verifying that it is valid JSON and with expected properties. Could maybe even check something more such as absence of characters that don't render, or non-use of deprecated APIs.

WDYT?

[caugner]

Good idea, I opened #641 to validate the JSON. That should be sufficient.