From my comments in #420:

I think we very much need to write up template designs, type conventions, extension conventions. This information is typically not suitably covered by an auto-generated API reference, as it is distributed across classes - the generic template, and the conventions implemented in the concrete desendants. Sth like in sklearn dev guidelines could work, but I'd actually prefer to also have "formal" template definitions collected in one place. While the typing makes things clearer, it also makes the necessity to document the interface cleanly greater.

Opinions, @mloning, @TonyBagnall?

I'm not sure what the "final form" should be here, but I feel this needs to be manually written and curated, with any modification to template patterns and conventions.
Could live on the wiki, linked from user docs, or directly in the official docs.

Happy to contribute to this, but let's discuss first where it should live.