JSON schemas are not syntax highlighted in docs

[lukasgraf]

The responses of type application/json+schema in the documentation for the @types endpoint are currently not getting syntax highlighted (see response at the bottom of that page).

This is because the respective Pygments Lexer in Sphinx is just registered for the application/json MIME type, but not application/json+schema.

Especially because these are long and deeply nested, the would benefit from getting properly highlighted.