docs: add custom titlebar example (#44506)

* 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

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Anny Yang <anny@electronjs.org>

docs/fiddles/features/window-customization/custom-title-bar/starter-code/main.js

@@ -0,0 +1,10 @@
+const { app, BrowserWindow } = require('electron')
+
+function createWindow () {
+  const win = new BrowserWindow({})
+  win.loadURL('https://example.com')
+}
+
+app.whenReady().then(() => {
+  createWindow()
+})