Block deploy to netlify if docs output is empty

### Description

Our build action should fail if, after the build has run, the `_site/docs/` directory is empty - otherwise we end up deploying the site without the docs available.

This should be part of the GH Action. It is valid to run a build locally and not include the docs when developing on other parts of the site.

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate