Use App instead of app in all getting started guides

[ealmloff]

We should use App in all of the getting started platform guides to be more consistent with the hackernews guide. Some of the guides already use App, but some like the fullstack guide do not

[neonwuchang]

1. Does this refer only to occurrences of fn app( ) ?
2. Should the updation be limited to code examples used in the getting started part only?

[ealmloff]

1. Does this refer only to occurrences of fn app( ) ?
2. Should the updation be limited to code examples used in the getting started part only?

It would be nice if it was consistent across all the docs, but this issue is specifically about the platform getting started guides because the code you create in those guides can be used to start the hackernews guide. The hackernews guide references the App component with a capital A here so the platform getting started guides should create that component