Add draft for backup strategy

docs/backup.md

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+# Backups
+
+CodeOcean persists most data in the PostgreSQL database including submissions and the respective user files. Only binary files that cannot be serialized are stored in the `public/uploads` folder in dedicated sub-folders with the ID as folder name. Necessary files are temporarily extracted to the container's file system during execution but don't need to backed up separately.
+
+## Summary
+
+In order to back up CodeOcean, you should consider these locations:
+
+- `config/*.yml` and `config/*.yml.erb`
+- The values of [environment variables](environment_variables.md) set for the web server
+- PostgreSQL database as specified in `config/database.yml`
+- `public/uploads` and all sub-folders