Reinstate validation of JSON Schemas (and examples) in build process

[MikeRalphson]

This issue is a place-holder to ensure this action isn't forgotten.

If we want to validate OAS instances beyond compliance with our JSON schemas, we would need to make a choice on which validator(s) to use - this would not be an endorsement.

Can occur after the resolution of #1270 / #1236

[MikeRalphson]

Script to validate JSON Schema instances against JSON Schema metaschema has been written and tested. Will be included in PR for HTML spec / registry go live.

[MikeRalphson]

Also script to validate examples against OAS schema instances, ditto.

[MikeRalphson]

Bump. Should this be reworked into a GitHub action?

[Bellangelo]

@MikeRalphson I can make the script to run in a Github action.