Platform: Docs: Move example app into docs

[brandonroberts]

Create a page for the example app describing what it is and its important parts. Move the stanadlone application into the docs example applications. From there we can generate a live StackBlitz example.

Other information:

I would be willing to submit a PR for the docs :heart:

[ ] Yes (Assistance is provided if you need help submitting a pull request)
[ ] No

[SerkanSipahi]

I would like to create the description for the example app. I have some questions:
1.) Where should be the description located? Maybe here: https://github.com/ngrx/platform/blob/master/projects/example-app/README.md
2.) Do you have any branch name convention? maybe pr/description-example-app?

[brandonroberts]

Apologies for the delay. A description of the example app and what's in it would be a good start. I don't think we'll move it from where it is now. We'll take another look at the tooling in time so see if we can include snippets of it in the guide.

1. You can create an example-app.md in this folder https://github.com/ngrx/platform/tree/master/projects/ngrx.io/content/guide
2. No specific convention for branch names.
3. Add a link at the bottom of the navigation.json here to point to it https://github.com/ngrx/platform/tree/master/projects/ngrx.io/content/guide

[SerkanSipahi]

Perfect, hopefully this weekend i will start:)

[brandonroberts]

Not going to do this after all