Add the documentation for the CKEditor 5 API

Closes #381

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+# CKEditor 5 - JavaScript API
+
+WoltLab Suite 6.0 ships with a customized version of CKEditor 5 that is enriched by multiple plugins to integrate into the framework.
+The editor can be accessed through an abstraction layer and event system to interact with basic functionality and to alter its initialization process.
+
+## The `Ckeditor` API
+
+The `WoltLabSuite/Core/Component/Ckeditor` API offers multiple helper methods to interface with CKEditor and to insert and extract content.
+You can get access to the instance using the `getCkeditor()` and `getCkeditorById()` methods once the editor has been initialized.
+
+Please refer to the typings of this component to learn about its API methods.
+
+## The Event System for CKEditor 5
+
+The event system of CKEditor 5 was designed for actions from within the editor but not to interface with the “outside world”.
+In order to provide a deeper integration there is `WoltLabSuite/Core/Component/Ckeditor/Event` and its exported function `listenToCkeditor()`.
+
+`listenToCkeditor()` expects the source element of the editor which usually is the `<textarea>` for the content.
+There is also `dispatchToCkeditor()` which is considered to be an internal API used from within the editor.
+
+### Lifecycle of CKEditor 5
+
+The initialization of any CKEditor 5 instance is asynchronous and goes through multiple steps to collect the data needed to initialize the editor.
+
+1. `setupFeatures()` is an override to disable the available features of this instance. The list of features becomes immutable after this event.
+2. `setupConfiguration()` is called at the end of the configuration phase and allows changes to the `EditorConfig` before it is passed to CKEditor.
+3. The `ready()` is fired as soon as the editor has been fully initialized and provides the `Ckeditor` object to access the editor.
+4. `collectMetaData()` is used in inline editors to request the injection of additional payloads that should be included in the server request.
+5. `reset()` notifies that the editor is being fully reset and reliant components should perform a reset themselves.
+6. `destroy()` signals that the editor has been destroyed and is no longer available.
+
+### Custom BBCode Buttons in the Editor Toolbar
+
+Custom buttons in the editor can be injected by pushing them to the toolbar in `setupConfiguration()`.
+This is implicitly takes place for buttons that are registered through the BBCode system and are set to appear in the editor.
+
+```ts
+listenToCkeditor(element).setupConfiguration(({ configuration }) => {
+  configuration.woltlabBbcode.push({
+    icon: "fire;false",
+    name: "foo",
+    label: "Button label for foo",
+  });
+});
+```
+
+Clicks on the buttons are forwarded through the `bbcode()` event.
+The only data passed to the callback is the name of the BBCode that was invoked.
+
+The default action of the custom buttons is to insert the BBCode into the editor, including the ability to wrap the selected text in the BBCode tags.
+Every event listener registered to this event MUST return a boolean to indicate if it wants to suppress the insertion of the text BBCode, for example to open a dialog or trigger another action.
+
+```ts
+listenToCkeditor(element).ready(({ ckeditor }) => {
+  listenToCkeditor(element).bbcode(({ bbcode }) => {
+    if (bbcode !== "foo") {
+      return false;
+    }
+
+    // Do something when the button for `foo` was clicked.
+
+    return true;
+  });
+});
+```
+
+### Submit the Editor on Enter
+
+The editor supports the special feature flag `submitOnEnter` that can be enabled through `setupFeatures()`.
+Once enabled it changes the behavior of the Enter key to signal that the contents should be submitted to the server.
+
+```ts
+listenToCkeditor(element).setupFeatures(({ features }) => {
+  features.submitOnEnter = true;
+});
+```
+
+You can subscribe to the `submitOnEnter()` to be notified when the Enter key was pressed and the editor is not empty.
+The contents of the editor is provided through the `html` key.
+
+This mode does not suppress the insertion of hard breaks through Shift + Enter.
+
+```ts
+listenToCkeditor(element).submitOnEnter(({ ckeditor, html }) => {
+  // Do something with the resulting `html`.
+});
+```