Add message to `validate_pr()` notifying of files submitted to non validated directories

Additional relevant discussion

Right, I also wasn't totally clear about this but basically landed in a similar place to where you did:

Any file in (a subfolder of) model-output would have validate_model_file run on it, which also checks for the existence of a corresponding model metadata file but doesn't actually validate that model metadata file's contents. For these checks, the file_path is relative to model-output.
Similarly, any file in (a subfolder of??) model-metadata would have validate_model_metadata run on it. For these checks, the file_path is relative to model-metadata.
- Here, (a subfolder of) is unnecessary because we could already reject these files on the grounds that they're in the wrong place... or we can just pass them on to validate_model_metadata and let this check verify that they're in the right place
So, what to do about other files?
- The strategy we use for the covid hub is that any file in another location is not validated by either of these functions, but a message pops up on the pull request saying something like, "this pull request includes one or more files that are in a location other than model-output or model-metadata". This strategy hasn't always been great (we've had an occasional accidental merge of files in unexpected locations because the PR validations didn't fail).
- So, yes, another idea could be to make some kind of effort to detect whether or not each file that's part of a pull request is an output file or a metadata file and run the relevant checks on them... but that seems potentially tricky (not every yaml or csv file in a repo is one of those two)

_Originally posted by @elray1 in https://github.com/Infectious-Disease-Modeling-Hubs/hubValidations/pull/21#discussion_r1327483993_

hubverse-org / hubValidations

Add message to `validate_pr()` notifying of files submitted to non validated directories #26