ci: skip running on docs-only changes

ci: skip running on docs-only changes

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-including-and-excluding-paths

> If you define a path with the ! character, you must also define at least one
> path without the ! character. If you only want to exclude paths, use
> paths-ignore instead.
>
> The order that you define patterns matters:
>     A matching negative pattern (prefixed with !) after a positive match will
>     exclude the path.
>     A matching positive pattern after a negative match will include the path
>     again.

Even if some of the exluded paths *could* impact the build, generally it's a
waste of time to do mkosi builds on them. Let's skip to releave the builders a
bit.