docs: add custom titlebar example (#43693)

* docs: add custom titlebar example

* docs: add links and other small edits

* docs: add panel window docs

* docs: remove panel example

* docs: specify expected emphasis style

* docs: responding to feedback

* docs: fix section names in links

* docs: rework baseWindow note

* docs: making window customization its own section

* responding to feedback

docs/fiddles/features/window-customization/custom-title-bar/native-window-controls/main.js

@@ -0,0 +1,15 @@
+const { app, BrowserWindow } = require('electron')
+
+function createWindow () {
+  const win = new BrowserWindow({
+    // remove the default titlebar
+    titleBarStyle: 'hidden',
+    // expose window controlls in Windows/Linux
+    ...(process.platform !== 'darwin' ? { titleBarOverlay: true } : {})
+  })
+  win.loadURL('https://example.com')
+}
+
+app.whenReady().then(() => {
+  createWindow()
+})