build(aio): big move of docs related files (#14361)

All the docs related files (docs-app, doc-gen, content, etc)
are now to be found inside the `/aio` folder.

The related gulp tasks have been moved from the top level
gulp file to a new one inside the `/aio` folder.

The structure of the `/aio` folder now looks like:

```
/aio/
  build/         # gulp tasks
  content/       #MARKDOWN FILES for devguides, cheatsheet, etc
    devguides/
    cheatsheets/
  transforms/    #dgeni packages, templates, etc
  src/
    app/
    assets/
    content/    #HTML + JSON build artifacts produced by dgeni from /aio/content.
                #This dir is .gitignored-ed
  e2e/           #protractor tests for the doc viewer app
  node_modules/ #dependencies for both the doc viewer builds and the dgeni stuff
                #This dir is .gitignored-ed
  gulpfile.js   #Tasks for generating docs and building & deploying the doc viewer
```

Closes #14361

aio/e2e/app.e2e-spec.ts

@@ -0,0 +1,36 @@
+import { SitePage } from './app.po';
+
+describe('site App', function() {
+  let page: SitePage;
+
+  beforeAll(done => {
+    // Hack:  CI has been failing on first test so
+    // buying time by giving the browser a wake-up call.
+    // Todo:  Find and fix the root cause for flakes.
+    new SitePage().navigateTo().then(done);
+  });
+
+  beforeEach(() => {
+    page = new SitePage();
+  });
+
+  it('should show features text after clicking "Features"', () => {
+    page.navigateTo()
+      .then(() => {
+        return page.featureLink.click();
+      })
+      .then(() => {
+        expect(page.getDocViewerText()).toContain('Progressive web apps');
+      });
+  });
+
+  it('should convert code-example in pipe.html', () => {
+    page.navigateTo()
+      .then(() => {
+        return page.datePipeLink.click();
+      })
+      .then(() => {
+        expect(page.codeExample.count()).toBeGreaterThan(0, 'should have code-example content');
+      });
+  });
+});