Improve Central API docs

[lognaturel]

• The output when extended metadata can be requested is really confusing. It looks like the example is always the extended one. And then the schema shows two arrays with no indication of what they mean. I think ideally they would be shown as two separate example/schema pairs, like in Apiary previously. I think the issue might be in the conversion to OpenAPI. I also notice that the following example/schema pair sometimes has json/extended as the content type (e.g. a 403)
• It would be nice to style text surrounded by backticks in parameter tables (code)
• Build some light tooling to manage multiple versions and allow users to navigate between them.
• Run tests in CI