Update "Getting started" with create-single-spa

[Carlos Filoteo]

The "Getting Started" page should have a section that guides a user to:

1. create a root config
2. create a basic application (generic? or perhaps starting with single-spa-html?)
3. wiring up that application to the root-config

single-spa-playground kind of serves this purpose but I've helped a few people who do not understand what to do once their application is running there.

Opening this discussion here to get feedback on whether this should be a beginner-to-single-spa's starting point.

[Joel Denning]

I agree that this would be a lot better than the current page.