Refactor: Uncouple from HTTP and use data models

[lornajane]

What/Why/How?

When we created the Museum API, we stopped iterating at the HTTP data transfer level. This PR doesn't change much of the API description (removed phone numbers from ticket records) but takes a more RESTful approach of modelling entities.

Still needs work: since the ID fields are readonly, I ran into https://github.com/Redocly/redocly-cli/issues/1416 which marks the examples as invalid.

Check yourself

• [x] Code is linted
• [ ] Tested with redoc/reference-docs/workflows (internal)
• [ ] All new/updated code is covered with tests

Security

• [x] Security impact of change has been considered
• [x] Code follows company security practices and guidelines