feat(docs-infra): add maskable PWA icon (#39928)

This commit adds a [maskable PWA icon][1] to ensure it looks good on
different devices/platforms. It also improves the PWA Lighthouse score
(from 96 to 100) by passing the [maskable icon audit][2].

For reference, the maskable icon can be previewed [here][3].

[1]: https://web.dev/maskable-icon/
[2]: https://web.dev/maskable-icon-audit
[3]:
https://maskable.app/?demo=https://user-images.githubusercontent.com/8604205/100861028-f16c9d80-3499-11eb-9341-beba69988cfa.png

PR Close #39928

aio/src/pwa-manifest.json

@@ -11,6 +11,12 @@
       "src": "assets/images/favicons/favicon-144x144.png",
       "sizes": "144x144",
       "type": "image/png"
+    },
+    {
+      "src": "assets/images/favicons/favicon-144x144-maskable.png",
+      "sizes": "144x144",
+      "type": "image/png",
+      "purpose": "maskable"
     }
   ],
   "start_url": "/?utm_source=homescreen",