docs: add example for taking screenshot (#20531)

docs/fiddles/media/screenshot/take-screenshot/renderer.js

@@ -0,0 +1,43 @@
+const { desktopCapturer } = require('electron')
+const { screen, shell } = require('electron').remote
+
+const fs = require('fs')
+const os = require('os')
+const path = require('path')
+
+const screenshot = document.getElementById('screen-shot')
+const screenshotMsg = document.getElementById('screenshot-path')
+
+screenshot.addEventListener('click', (event) => {
+  screenshotMsg.textContent = 'Gathering screens...'
+  const thumbSize = determineScreenShotSize()
+  const options = { types: ['screen'], thumbnailSize: thumbSize }
+
+  desktopCapturer.getSources(options, (error, sources) => {
+    if (error) return console.log(error)
+
+    sources.forEach((source) => {
+      const sourceName = source.name.toLowerCase()
+      if (sourceName === 'entire screen' || sourceName === 'screen 1') {
+        const screenshotPath = path.join(os.tmpdir(), 'screenshot.png')
+
+        fs.writeFile(screenshotPath, source.thumbnail.toPNG(), (error) => {
+          if (error) return console.log(error)
+          shell.openExternal(`file://${screenshotPath}`)
+
+          const message = `Saved screenshot to: ${screenshotPath}`
+          screenshotMsg.textContent = message
+        })
+      }
+    })
+  })
+})
+
+function determineScreenShotSize () {
+  const screenSize = screen.getPrimaryDisplay().workAreaSize
+  const maxDimension = Math.max(screenSize.width, screenSize.height)
+  return {
+    width: maxDimension * window.devicePixelRatio,
+    height: maxDimension * window.devicePixelRatio
+  }
+}