docs: add notification examples (#20528)

Refs #20442

Adds the basic notification and notification with custom image examples from electron-api-demos into runnable Fiddle examples.

Gist links to Fiddles (same as code submitted in this PR):
Basic Notification: https://gist.github.com/102945f83f559e7944797175d8fd8af4
Notification with image: https://gist.github.com/2688bf4bfc27ce02f5d74224828eb928

Co-Authored-By: Erick Zhao <erick@hotmail.ca>

docs/fiddles/native-ui/notifications/basic-notification/main.js

@@ -0,0 +1,25 @@
+const { BrowserWindow, app } = require('electron')
+
+let mainWindow = null
+
+function createWindow () {
+  const windowOptions = {
+    width: 600,
+    height: 300,
+    title: 'Basic Notification',
+    webPreferences: {
+      nodeIntegration: true
+    }
+  }
+
+  mainWindow = new BrowserWindow(windowOptions)
+  mainWindow.loadFile('index.html')
+
+  mainWindow.on('closed', () => {
+    mainWindow = null
+  })
+}
+
+app.on('ready', () => {
+  createWindow()
+})