Added custom bundles, packaging material, pickup points and returns

We updated the Create a shipment guide with details about:

• How to send custom bundles in the request
• How to suggest the warehouse the packaging material to be used in the parcel
• How to specify pickup points for DHL Germany
• How to indicate Cash On Delivery (CoD) shipments

We updated the Listing Returns guide with a wider explanation about the different return types and reasons.