[DOCS] Add JS custom event documentation

Describe the given JS custom events by
listing the given events and how to listen
to them.

Documentation/Developer/Index.rst

@@ -13,3 +13,4 @@ Information for developer how to extend the extension.
 
    Events/Index
    Hooks/Index
+   JavaScript-Events/Index

Documentation/Developer/JavaScript-Events/Index.rst

@@ -0,0 +1,51 @@
+.. include:: ../../Includes.txt
+
+=================
+JavaScript Events
+=================
+
+Multiple `custom JavaScript events<https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent>`__
+allow to react with own JavaScript on JavaScript which is executed after certain
+user actions.
+
+
+List of events
+==============
+The following events exists:
+
+=========================================== ====================================
+Event name                                  User action which triggers the event
+=========================================== ====================================
+`extcode:be-variant-was-changed`            A BE variant of a product was chosen.
+`extcode:country-updated`                   The country of billing or shipping address is updated.
+`extcode:currency-updated`                  The currency of the cart is updated.
+`extcode:hide-message-block`                The success/error message after adding a product to the cart will be hidden.
+`extcode:minicart-was-updated`              A product is added to the cart (which updates the mini cart).
+`extcode:render-add-to-cart-result-message` A product is added to the cart which renders a success (or error) message.
+`extcode:set-payment`                       Another payment option is chosen.
+`extcode:set-shipping`                      Another shipping option is chosen.
+=========================================== ====================================
+
+.. TIP::
+   The best way to understand when the event is executed and which data it
+   contains can be seen in it's context. Search in the directory
+   `/Build/Sources/` for the event name.
+
+How to listen for events
+========================
+
+Every event comes with a detail object which contains data relevant for this
+specific event.
+
+The following example shows how to listen for an event.
+
+.. code-block:: javascript
+   :caption: Extract of some JavaScript added with e.g. an AssetViewhelper
+   document.addEventListener("extcode:country-updated", (event) => {
+     console.log("The event 'extcode:country-updated' was fired.");
+     console.log("The event contains the following detail data:");
+     console.log(event.detail);
+   });
+
+
+