docs(storage): clarify file mount prerequisites

[vaguul]

Changes

• Add file bind mount prerequisites to the Docker Compose storage docs.
• Add troubleshooting guidance for not a directory: unknown mount failures.
• Add a matching Persistent Storage warning for file bind mounts.

Issue

• Closes Add user education about file mount prerequisites and troubleshooting #347

Verification

• Checked the Coolify UI label for Preserve Repository During Deployment in the application settings.
• Checked Coolify deployment code paths that use the preserve repository setting for Docker Compose deployments.

Testing

• git diff --check
• Not run: local docs build/typecheck, because this checkout has no bun command or node_modules installed.