Add screenshot to the Get Started / Create an App page

[rq-aszasz]

Link to doc page in question (if any):
https://github.com/streamlit/docs/blob/main/content/library/get-started/create-an-app.md

Name of the Streamlit feature whose docs need improvement:

What you think the docs should say:
An app screenshot would give a good idea about the app structure described.

[sfc-gh-dmatthews]

The Get started section is in the process of getting revamped. This should be addressed soon with those updates, likely with an live, embedded example of the app. Thanks!