There's an example API created with the service here: https://sheetlabs.com/#/services/doc/ACME/getDomain

This is linked off the front page, but could certainly be more prominent (as another commenter said)

I stared at that for 10 minutes trying to find the spreadsheet. I assume there is a spreadsheet backing it? You should actually show people the spreadsheet - just seeing an API description without demonstrating that it runs off a simple spreadsheet almost kind of defeats the whole purpose of the service!

Thanks for the feedback. Yes, it is backed onto a spreadsheet effectively. Showing it is a good idea for the front page.

But I don't think showing the spreadsheet in the API docs is so useful; the idea is to present a clean API to the users, and they don't even need to know that the data was originally from a spreadsheet.

+1 on showing the spreadsheet. It doesn't have to be on the docs, can be on the website. I'd put front and center, if you have this spreadsheet, it creates this API. You can even look into giving access to the sample spreadsheet for testing directly from the web pages.