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-window-styles/transparent-windows/main.js

@@ -0,0 +1,16 @@
+const { app, BrowserWindow } = require('electron')
+
+function createWindow () {
+  const win = new BrowserWindow({
+    width: 100,
+    height: 100,
+    resizable: false,
+    frame: false,
+    transparent: true
+  })
+  win.loadFile('index.html')
+}
+
+app.whenReady().then(() => {
+  createWindow()
+})