When in doubt, ask Stripe: https://stripe.com/docs

Not a company as such, but Django's documentation is very well written. Jacob Kaplan-Moss has a really great article about it (part of a series) which I think sums up what makes it so great.

The layout of the docs could be better, though, especially when it comes to the reference documentation. I find that I want to jump around a lot when I'm reading the Django reference docs, and the navigation doesn't really accommodate that as well as, say, Stripe (which has already been mentioned). The downside of the Stripe layout, though, is that the reference is so much different to the prose documentation, so it feels weird switching between the two.

I made my own.... some functionality is missing - but I would like to build a CMS on it.

https://simonfricker.github.io/apiDocs/

Here are a few API docs that I really like

Mollie

Plaid

Digital Ocean

Mapbox

Mailchimp