Add warning message in `files` folder to avoid accidental deletion
Add warning message in `files` folder to avoid accidental deletion

This is a pretty standard practice for applications that have data
stored in folders where a user may accidentally determine that the
content is unnecessary.

Aims to address cases like
https://github.com/ppy/osu/discussions/20394#discussioncomment-3705694.
It's not the first time this has come up, and definitely won't be the
last.

peppy/osu,peppy/osu,peppy/osu,ppy/osu,ppy/osu,ppy/osu