docs: Document Rajaongkir integration issue and add session support

## Discovery ✅

Rajaongkir plugin uses a completely different approach:
- Removes standard WooCommerce city/state fields
- Adds custom destination dropdown with Select2 search
- Stores destination in WooCommerce session (not address fields)
- Reads from session during shipping calculation

## Root Cause of Issues:

### 1. Same rates for different provinces
- OrderForm sends: city="Bandung", state="Jawa Barat"
- Rajaongkir ignores these fields
- Rajaongkir reads: WC()->session->get("selected_destination_id")
- Session empty → Uses cached/default rates

### 2. No Rajaongkir API hits
- No destination_id in session
- Rajaongkir can't calculate without destination
- Returns empty or cached rates

## Backend Fix (✅ DONE):

Added Rajaongkir session support in calculate_shipping:
```php
// Support for Rajaongkir plugin
if ( $country === 'ID' && ! empty( $shipping['destination_id'] ) ) {
    WC()->session->set( 'selected_destination_id', $shipping['destination_id'] );
    WC()->session->set( 'selected_destination_label', $shipping['destination_label'] );
}
```

## Frontend Fix (TODO):

Need to add Rajaongkir destination field:
1. Add destination search component (Select2/Combobox)
2. Search Rajaongkir API for locations
3. Pass destination_id to backend
4. Backend sets session before calculate_shipping()

## Documentation:

Created RAJAONGKIR_INTEGRATION.md with:
- How Rajaongkir works
- Why our implementation fails
- Complete solution steps
- Testing checklist

## Next Steps:

1. Add Rajaongkir search endpoint to OrdersController
2. Create destination search component in OrderForm
3. Pass destination_id in shipping data
4. Test with real Rajaongkir API

includes/Api/OrdersController.php

@@ -1312,6 +1312,17 @@ class OrdersController {
         WC()->customer->set_billing_state( $state );
         WC()->customer->set_billing_postcode( $postcode );
         WC()->customer->set_billing_city( $city );
+        
+        // Support for Rajaongkir plugin - set destination in session
+        // Rajaongkir uses session-based destination instead of standard address fields
+        if ( $country === 'ID' && ! empty( $shipping['destination_id'] ) ) {
+          WC()->session->set( 'selected_destination_id', $shipping['destination_id'] );
+          WC()->session->set( 'selected_destination_label', $shipping['destination_label'] ?? $city );
+        } else {
+          // Clear Rajaongkir session data for non-ID countries
+          WC()->session->__unset( 'selected_destination_id' );
+          WC()->session->__unset( 'selected_destination_label' );
+        }
       }
 
       // Calculate shipping