Provide alternate API spec variants

[leonhard-s]

The reference spec will be technically correct and cover both the source and cast type for all fields, as well as provide any documentation required to use the API.

However, this compromises all use cases. Instead, we could provide different "releases" of the same version of the spec, for different deployments:

• The documentation spec on ReadTheDocs could use the cast types for readability, rather than typing everything as string (even if this is correct)
• Client code may want a smaller, lower-bandwidth version of the spec that does away with examples and documentation
• There could still be use cases for which a full monolith is better suited than the multi-file layout the repo will use/should be using