Lexical API: Started working on docs format and jest testing

dev/docs/wysiwyg-js-api.md

@@ -5,4 +5,93 @@ TODO - Create JS events and add to the js public events doc.
 
 TODO - Document the JS API.
 
-TODO - Add testing coverage
+TODO - Add testing coverage
+
+**Warning: This API is currently in development and may change without notice.**
+
+This document covers the JavaScript API for the (newer Lexical-based) WYSIWYG editor.
+This API is custom-built, and designed to abstract the internals of the editor away
+to provide a stable interface for performing common customizations.
+
+Only the methods and properties documented here are guaranteed to be stable **once this API
+is out of initial development**.
+Other elements may be accessible but are not designed to be used directly, and therefore may change
+without notice.
+Stable parts of the API may still change where needed, but such changes would be noted as part of BookStack update advisories.
+
+## Overview
+
+The API is provided as an object, which itself provides a number of modules
+via its properties:
+
+- `ui` - Provides all actions related to the UI of the editor, like buttons and toolbars.
+
+Each of these modules, and the relevant types used within, can be found detailed below.
+
+---
+
+## UI Module
+
+This module provides all actions related to the UI of the editor, like buttons and toolbars.
+
+### Methods
+
+#### createButton(options)
+
+Creates a new button which can be used by other methods.
+This takes an option object with the following properties:
+
+- `label` - string, optional - Used for the button text if no icon provided, or the button tooltip if an icon is provided.
+- `icon` - string, optional - The icon to use for the button. Expected to be an SVG string.
+- `action` - callback, required - The action to perform when the button is clicked.
+
+The function returns an [EditorApiButton](#editorapibutton) object.
+
+**Example**
+
+```javascript
+const button = api.ui.createButton({
+    label: 'Warn',
+    icon: '<svg>...</svg>',
+    action: () => {
+        window.alert('You clicked the button!');
+    }
+});
+```
+
+### getMainToolbarSections()
+
+Get the sections of the main editor toolbar. These are those which contain groups of buttons
+with overflow control.
+
+The function returns an array of [EditorToolbarSection](#editortoolbarsection) objects.
+
+**Example**
+
+```javascript
+const sections = api.ui.getMainToolbarSections();
+if (sections.length > 0) {
+    sections[0].addButton(button);
+}
+```
+
+### Types
+
+These are types which may be provided from UI module methods.
+The methods on these types are documented using standard TypeScript notation.
+
+#### EditorApiButton
+
+Represents a button created via the `createButton` method.
+This has the following methods:
+
+- `setActive(isActive: boolean): void` - Sets whether the button should be in an active state or not (typically active buttons appear as pressed).
+
+#### EditorToolbarSection
+
+Represents a section of the main editor toolbar, which contains a set of buttons.
+This has the following methods:
+
+- `getLabel(): string` - Gets the label of the section.
+- `addButton(button: EditorApiButton, targetIndex: number = -1): void` - Adds a button to the section.
+  - By default, this will append the button, although a target index can be provided to insert the button at a specific position.