build(aio): implement prerendering

The current implementation is based on @igorminar's [angular-io-v42][1]. It is
using Protractor to request all docs URLs, let them fallback to `/index.html`
and save the rendered page.

[1]: https://github.com/IgorMinar/angular-io-v42/tree/05508ab3/tools/prerenderer

Fixes #15104

aio/tools/prerender/constants.js

@@ -0,0 +1,30 @@
+'use strict';
+
+// Imports
+const path = require('path');
+
+// Constants
+const BROWSER_INSTANCES = 7;
+
+const PORT = 4201;
+const BASE_URL = `http://localhost:${PORT}`;
+
+const ROOT_DIR = path.join(__dirname, '../..');
+const DIST_DIR = path.join(ROOT_DIR, 'dist');
+const CONTENT_DIR = path.join(DIST_DIR, 'content');
+const INPUT_DIR = path.join(CONTENT_DIR, 'docs');
+const TMP_SPECS_DIR = path.join(ROOT_DIR, 'tmp/docs-prerender-specs');
+const TMP_OUTPUT_DIR = path.join(ROOT_DIR, 'tmp/docs-prerendered');
+
+// Exports
+module.exports = {
+  BASE_URL,
+  BROWSER_INSTANCES,
+  CONTENT_DIR,
+  DIST_DIR,
+  INPUT_DIR,
+  PORT,
+  ROOT_DIR,
+  TMP_OUTPUT_DIR,
+  TMP_SPECS_DIR
+};