Lazy load documentation modules

[Splaktar]

This PR implements the first round of lazy loading for the site. In general, the only significant impact is on the bundle size reduction for the home page and the guide list.

The guides themselves should also see a significant reduction in bundle size, but they depend upon the DocViewer which depends on the ExampleViewer which pulls in the full @angular/components-examples along with every Angular Material and CDK component.

This is the first step towards solving #176, which calls out the concerns with ExampleViewer. ExampleViewer isn't tied to a route, so we can't lazy load it the normal way.

Most Guide's only need the DocViewer, but the following 3 guides do actually require the ExampleViewer:

• Creating a custom form field control
• Using elevation helpers
• Creating a custom stepper using the CdkStepper

Further investigation is needed to

• determine the best way to de-couple these two viewer components
• determine the best approach for lazy loading the ExampleViewer
• change the ComponentViewer to only import the components and examples needed for the current component

Impacts

Listed as compressed bundle size on page load with cache disabled in Chrome ("transferred").

Homepage

Before: 600 KB
After: 331 KB
✅ 45% reduction

Guide List

Before: 594 KB
After: 342 KB
✅ 42% reduction

Guide Viewer

Before: 618 KB
After: 640 KB
🚧 3% increase

Component Docs

Before: 627 KB
After: 652 KB
🚧 4% increase

[jelbourn]

LGTM