Clarify status of sample app in stackblitz

[jbogarthyde]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

https://angular.io/start Get-started guide does not make it clear that the sample app is a ready-made illustrative teaching app, not the generic app you get from the StackBlitz "Create Angular App" button.

Issue Number: #33213

What is the new behavior?

Clarifies status of teaching app, mentions availability of generic app builder, and compares StackBlitz as a learning environment to CLI as a more advanced development tool.

Does this PR introduce a breaking change?

• Yes
• No

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[kapunahelewong]

LGTM! 🎉

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