Revamp screenshots in Supported Platforms

What kind of documentation issue is this?

Incorrect documentation

Affected page

https://toga.beeware.org/en/latest/reference/platforms/

Describe the issue

At https://toga.beeware.org/en/latest/reference/platforms/, if you click on all the platforms, Qt is missing a screenshot for the demo app; Windows screenshot uses Win10 style; and macOS screenshot uses an extremely ancient style.

Suggested improvement

Update screenshots for all desktop platforms; mobile and web/textual doesn't have enough widgets or structures to run the demo app yet. Align the screenshots with the homepage screenshots as well.