New endpoint for categories list about api

[sevko]

I have two concerns about this:

• would it be an expensive query? Even though elasticsearch must maintain some smart internal indexes, I'd imagine that an aggregation query might take a long time, and exposing that as a readily hittable API endpoint feels a bit.. dangerous. That being said, we can probably cache the result once and refresh only when new records have been indexed.
• is this necessary? Since the taxonomy should be fairly static, I feel like it should be kept in API documentation rather exposed via an endpoint.

[missinglink]

I would be 👍 for documenting this via a human readable method rather than a machine-readable one, the latter would not be very useful without the former.