docs: add Synchronous Messages Fiddle example (#20451)

* docs: add Synchronous Messages Fiddle example

* Code review changes

* Add OS support info

docs/fiddles/communication/two-processes/synchronous-messages/main.js

@@ -0,0 +1,29 @@
+const { app, BrowserWindow, ipcMain } = require('electron')
+
+let mainWindow = null
+
+function createWindow () {
+  const windowOptions = {
+    width: 600,
+    height: 400,
+    title: 'Synchronous Messages',
+    webPreferences: {
+      nodeIntegration: true
+    }
+  }
+
+  mainWindow = new BrowserWindow(windowOptions)
+  mainWindow.loadFile('index.html')
+
+  mainWindow.on('closed', () => {
+    mainWindow = null
+  })
+}
+
+app.on('ready', () => {
+  createWindow()
+})
+
+ipcMain.on('synchronous-message', (event, arg) => {
+    event.returnValue = 'pong'
+})