showcase: lazy load ExampleViewerModule after login

[SuperITMan]

I'm submitting a...

[X] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Due to #2953, first navigation to a page under "Stark UI", "Stark Core", "Stark RBAC"... takes a while since the ExampleViewerModule is downloaded just before opening the page.
The bundle of 10MB is downloaded at that time.

Expected behavior

The ExampleViewerModule module should be lazyloaded before accessing the pages, since we are sure that these pages will be accessed.
We could load the module once the home page has been loaded.

Minimal reproduction of the problem with instructions

Start the showcase, open the DevTools and access to a page in "Stark UI", check the Network tab.

What is the motivation / use case for changing the behavior?

Fix UX after improvement thanks to the split of the stark-ui chunks.

Environment

Angular version: 12.x
Stark version: 12.x