API documentation

Look through LaunchAny / John Higginbottom for information on good documentation

also: do not introduce terms unique to you (like Zuora does)

textline/apiary --- don't use apiary for docs

https://loopback.io/doc/en/lb3/Use-API-Explorer.html

https://swagger.io/

FYI, in case you aren’t using loopback, the api explorer that they use is built on top of swagger, and you can still get most of the benefits from configuring swagger to map to whatever api backend you’re using

https://firebase.google.com/docs/firestore/

https://reference.chargify.com/v1/billing-portal/read-the-management-link

https://hackernoon.com/how-to-write-great-api-documentation-c710cd1c696

has other links for more learning (they suggest stripe's API)
detailed examples
quick to get started & with things like
make available offline --- that's a benefit of one-page documentation

Other ones recommended:
Slate
Readme.io
Swagger/OpenAPI/WidderShins