Add documentation / screenshots for the "Webhook ID" value

There is new documentation in the pipeline, but this commit adds more information to the UX.

Steps to make use of webhooks:

1. Go to the edit form of the PayPal Payment Gateway of your site.
2. Copy the Webhook Url from there. It is: https://[domain-name]/payment/notify/[payment-gateway-machine-name], but the gateway edit form will take care of displaying this for you.
3. Take note of the 5 events that are supported.
4. Go to your PayPal developer dashboard, and edit your site's application.
5. Choose to add a new Webhook.
6. Specify the URL you got from the Payment Gateway, and enable the 5 events specified.
7. Copy the resulting Webhook Id.
8. Add the Webhook Id to your PayPal Payment Gateway, and save.

The two primary benefits for the Webhook:

1. On rare occasions, PayPal may return a pending, rather than completed status on a payment during checkout. Checkout will still complete, but the order has not technically be paid in full until a webhook notification tells us so. This does vary per merchant and customer, so some sites might never encounter this, while others may have a higher frequency.
2. If a payment is refunded through the PayPal UX, rather than the site UX, the webhook will send a notification, so that the payment refund is reflected on the order. Note that if this is done by the merchant, it is important to define what the workflow is. e.g. is there a shipment that needs to be pulled? Is there a license that needs to be cancelled? etc...

If a merchant is not having issues with pending payments, and is not doing refunds from the Site UX, then using Webhooks might not be needed.

Thank you very very much for the work on this @TomTech great to see activity here! Thank you :)

Automatically closed - issue fixed for 2 weeks with no activity.