Update documentation for Shakapacker 9.0+ private_output_path integration

Documents the recommended approach of using Shakapacker 9.0+ private_output_path
for server bundle configuration, providing a single source of truth.

**Documentation Updates:**

**1. Configuration API Reference (docs/api-reference/configuration.md)**
- Added prominent recommendation for Shakapacker 9.0+ approach
- Documents shakapacker.yml private_output_path configuration
- Explains auto-detection behavior
- Preserves documentation for older versions

**2. Webpack Configuration Guide (docs/core-concepts/webpack-configuration.md)**
- New section: "Server Bundle Configuration (Shakapacker 9.0+)"
- Complete example with shakapacker.yml and webpack config
- Lists benefits of the new approach:
  - Single source of truth
  - Automatic synchronization
  - No configuration duplication
  - Better maintainability
- Notes compatibility with older versions

**Key Points:**
- Shakapacker 9.0+ users get automatic configuration
- Backward compatible with manual configuration
- Generator templates already show both approaches
- Doctor command guides users to upgrade

**Related Changes:**
- Generator templates already updated in previous commit
- Auto-detection implemented in configuration.rb
- Doctor provides version-aware recommendations

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

docs/api-reference/configuration.md

@@ -126,6 +126,15 @@ ReactOnRails.configure do |config|
   # SERVER BUNDLE SECURITY AND ORGANIZATION
   ################################################################################
 
+  # ⚠️ RECOMMENDED: Use Shakapacker 9.0+ for Automatic Configuration
+  #
+  # For Shakapacker 9.0+, add to config/shakapacker.yml:
+  #   private_output_path: ssr-generated
+  #
+  # React on Rails will automatically detect and use this value, eliminating the need
+  # to configure server_bundle_output_path here. This provides a single source of truth.
+  #
+  # For older Shakapacker versions or custom setups, manually configure:
   # This configures the directory (relative to the Rails root) where the server bundle will be output.
   # By default, this is "ssr-generated". If set to nil, the server bundle will be loaded from the same
   # public directory as client bundles. For enhanced security, use this option in conjunction with

docs/core-concepts/webpack-configuration.md

@@ -78,6 +78,38 @@ default: &default
 
 The `bin/switch-bundler` script automatically updates this configuration when switching bundlers.
 
+### Server Bundle Configuration (Shakapacker 9.0+)
+
+**Recommended**: For Shakapacker 9.0+, use `private_output_path` in `shakapacker.yml` for server bundles:
+
+```yaml
+default: &default # ... other config ...
+  private_output_path: ssr-generated
+```
+
+This provides a single source of truth for server bundle location. React on Rails automatically detects this configuration, eliminating the need to set `server_bundle_output_path` in your React on Rails initializer.
+
+In your `config/webpack/serverWebpackConfig.js`:
+
+```javascript
+const { config } = require('shakapacker');
+
+serverWebpackConfig.output = {
+  filename: 'server-bundle.js',
+  globalObject: 'this',
+  path: config.privateOutputPath, // Automatically uses shakapacker.yml value
+};
+```
+
+**Benefits:**
+
+- Single source of truth in `shakapacker.yml`
+- Automatic synchronization between webpack and React on Rails
+- No configuration duplication
+- Better maintainability
+
+**For older Shakapacker versions:** Use hardcoded paths and manual configuration as shown in the generator templates.
+
 Per the example repo [shakacode/react_on_rails_demo_ssr_hmr](https://github.com/shakacode/react_on_rails_demo_ssr_hmr),
 you should consider keeping your codebase mostly consistent with the defaults for [Shakapacker](https://github.com/shakacode/shakapacker).