fix(aio): improve transitions between pages

- Avoid unnecessary animations, style transitions, repositioning on
  initial rendering.
- Better handle transitioning from/to Home page (which is the only page
  with transparent top-menu).
- Better coordinate sidenav and hamburger animations with page
  transitions.
- Improve fade-in/out animations.

Fixes #20996

aio/src/app/layout/doc-viewer/doc-viewer.component.spec.ts

@@ -368,7 +368,7 @@ describe('DocViewerComponent', () => {
       });
 
       it('should display nothing if the document has no contents', async () => {
-        docViewer.currViewContainer.innerHTML = 'Test';
+        await doRender('Test');
         expect(docViewerEl.textContent).toBe('Test');
 
         await doRender('');
@@ -647,6 +647,8 @@ describe('DocViewerComponent', () => {
       oldCurrViewContainer.innerHTML = 'Current view';
       oldNextViewContainer.innerHTML = 'Next view';
 
+      docViewerEl.appendChild(oldCurrViewContainer);
+
       expect(docViewerEl.contains(oldCurrViewContainer)).toBe(true);
       expect(docViewerEl.contains(oldNextViewContainer)).toBe(false);
     });