feat(docs): add developer guide

docs/Developer Guide/Developer Guide/Scripting/Widgets/CSS.md

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+# CSS
+In `doRender()`:
+
+```plain
+this.cssBlock(`#my-widget {
+	position: absolute;
+    bottom: 40px;
+    left: 60px;
+    z-index: 1;
+}`)
+```
+
+* * *
+
+Reference: [https://trilium.rocks/X7pxYpiu0lgU](https://trilium.rocks/X7pxYpiu0lgU)

docs/Developer Guide/Developer Guide/Scripting/Widgets/Right pane widget.md

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+# Right pane widget
+*   `doRender` must not be overridden, instead `doRenderBody()` has to be overridden.
+*   `parentWidget()` must be set to `“rightPane”`.
+*   `widgetTitle()` getter can optionally be overriden, otherwise the widget will be displayed as “Untitled widget”.
+
+```plain
+const template = `<div>Hi</div>`;
+
+class ToDoListWidget extends api.RightPanelWidget {
+    
+    get widgetTitle() {
+        return "Title goes here";
+    }
+        
+    get parentWidget() { return "right-pane" }
+    
+    doRenderBody() {
+        this.$body.empty().append($(template));
+    }   
+    
+    async refreshWithNote(note) {
+        this.toggleInt(false);                
+        this.triggerCommand("reEvaluateRightPaneVisibility");
+        this.toggleInt(true);
+        this.triggerCommand("reEvaluateRightPaneVisibility");
+    }
+}
+
+module.exports = new ToDoListWidget();
+```
+
+The implementation is in `src/public/app/widgets/right_panel_widget.js`.