This should be a Markdown cell. It is not displaying correctly as HTML.

Changed to Markdown cell; thanks, Mike.

We should recommend Inputs.range here instead of a native HTML input.

Changed to use Inputs.range; thanks, Mike.

I think API keys notebook should be in "Editing, Publishing and Collaborating" instead of "Working with Data" since its about exporting notebooks using the Observable API

Moved, thanks, Ian!

I was searching for "invalidation", a key cell-related concept, but search did not find it.

Thanks, Claudio; we've added the Invalidation notebook to the JavaScript in Observable collection, so you should be able to find it now.