fix(aio): fix scrolling to top (#17102)

Previously, the `#top-of-page` element (used when scrolling to top) was placed
inside the content section (which at the time had zero top margin and padding).
Furthermore, there was a top offset applied when scrolling that took the static
top bar's height into account. Since now the top bar is not static any more and
the content section has a non-zero top padding, scrolling to top does not work
as expected.

This commit fixes this by:
- Moving the `#top-of-page` element to the top of the `aio-shell`.
- Stop accounting for the top bar's top.

Fixes #17006

aio/e2e/app.e2e-spec.ts

@@ -41,6 +41,26 @@ describe('site App', function() {
     expect(page.getInnerHtml(codeExample)).toContain('<h1>Tour of Heroes</h1>');
   });
 
+  describe('scrolling to the top', () => {
+    it('should scroll to the top when navigating to another page', () => {
+      page.navigateTo('guide/docs');
+      page.scrollToBottom();
+      page.getScrollTop().then(scrollTop => expect(scrollTop).toBeGreaterThan(0));
+
+      page.navigateTo('guide/api');
+      page.getScrollTop().then(scrollTop => expect(scrollTop).toBe(0));
+    });
+
+    it('should scroll to the top when navigating to the same page', () => {
+      page.navigateTo('guide/docs');
+      page.scrollToBottom();
+      page.getScrollTop().then(scrollTop => expect(scrollTop).toBeGreaterThan(0));
+
+      page.navigateTo('guide/docs');
+      page.getScrollTop().then(scrollTop => expect(scrollTop).toBe(0));
+    });
+  });
+
   describe('api-docs', () => {
     it('should show a link to github', () => {
       page.navigateTo('api/common/NgClass');