404 Error Handler?

Where do you put the 404 error handling page for a site?

Something like /errors/404.html.twig? Or is there a prescribed way?

404 errors are handled by the errors plugin. That plugin provides it's own error.html.twig. You can however provide your own, it would just need to be in the templates folder of your theme.

I have that working now. Does errors handle any other errors beyond 404?

What kind of errors do you have in mind? Are you talking about server errors? Those are now t handled by this plugin, they are handled internally by Grav.

OK, some maybe it should just be 404.html.twig and 404.md?

Well, not really because you can use it for custom codes, by setting a http_response_code value in the page.

I saw http_response_code but wasn't sure what it did. Now I know!

It is documented although admittedly not much detail :)