docs: update Quick Start Guide for Electron 12 (#28223)

* docs: Update Quick Start Guide for Electron 12

With `contextIsolation` enabled by default in Electron 12, the Getting Started Guide no longer works as it is written. In order for the basic example to display values from `process.versions`, we need to add a `preload.js` to the example.

* fix: annotate preload code block with a language

* docs: update quick-start Fiddle example to use preload to provide version info

* fix: ensure example files end in a newline

* docs: add security warning to instructions for turning off contextIsolation

Co-authored-by: John Kleinschmidt <jkleinsc@github.com>

* docs: treat preload as an adjective instead of a noun

Co-authored-by: John Kleinschmidt <jkleinsc@github.com>

Co-authored-by: John Kleinschmidt <jkleinsc@github.com>

docs/fiddles/quick-start/preload.js

@@ -0,0 +1,11 @@
+window.addEventListener('DOMContentLoaded', () => {
+  const replaceText = (selector, text) => {
+    const element = document.getElementById(selector)
+    if (element) element.innerText = text
+  }
+
+  for (const type of ['chrome', 'node', 'electron']) {
+    replaceText(`${type}-version`, process.versions[type])
+  }
+})
+