docs: modernize protocol-handler docs (#29380)

* docs: modernize protocol-handler docs

* docs: iadd contextIsolation

* docs: add guide for launch-app-from-URL-in-other-app

* docs: address comments

* chore: fix brackets

* chore: add escaped brackets

docs/fiddles/system/protocol-handler/launch-app-from-URL-in-another-app/preload.js

@@ -0,0 +1,11 @@
+// All of the Node.js APIs are available in the preload process.
+// It has the same sandbox as a Chrome extension.
+const { contextBridge, ipcRenderer } = require('electron')
+
+// Set up context bridge between the renderer process and the main process
+contextBridge.exposeInMainWorld(
+  'shell',
+  {
+    open: () => ipcRenderer.send('shell:open'),
+  }
+)