Add private_output_path to shakapacker.yml generator template

Adds commented-out private_output_path configuration to the generated
shakapacker.yml, making it easy for users to enable the Shakapacker 9.0+
integration feature.

**Changes:**
- Added private_output_path: ssr-generated (commented out)
- Clear documentation that it's a Shakapacker 9.0+ feature
- Notes that React on Rails automatically detects it
- Positioned logically after public_output_path configuration

**User Experience:**
Users can now simply uncomment one line in shakapacker.yml to enable
automatic configuration instead of needing to manually add it.

**Complete Integration:**
- ✅ Auto-detection in configuration.rb
- ✅ Doctor recommendations
- ✅ Generator templates (webpack + initializer)
- ✅ Shakapacker.yml template (NEW)
- ✅ Documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

lib/generators/react_on_rails/templates/base/base/config/shakapacker.yml

@@ -29,6 +29,12 @@ default: &default
   # Location for manifest.json, defaults to {public_output_path}/manifest.json if unset
   # manifest_path: public/packs/manifest.json
 
+  # Location for private server-side bundles (e.g., for SSR)
+  # These bundles are not served publicly, unlike public_output_path
+  # Shakapacker 9.0+ feature - automatically detected by React on Rails
+  # Uncomment to enable (requires Shakapacker 9.0+):
+  # private_output_path: ssr-generated
+
   # Additional paths webpack should look up modules
   # ['app/assets', 'engine/foo/app/assets']
   additional_paths: []