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.

CORE TEAM EDIT: The `screenshot` app in the examples folder of the Toga repo contains an app that will auto-generate *most* of the screenshots (with some manual intervention for a handful of widgets that can't be automatically captured). That app needs to be run on a *vanilla* install of each operating system - no colour or theme customizations, or dark mode enabled. On Linux, that means a *vanilla* Ubuntu install for GTK, or Kubuntu for Qt.

Start with the Supported Platforms page and the `screenshot` app in the examples folder. Run it on vanilla installs for each desktop platform, using vanilla Ubuntu for GTK and Kubuntu for Qt, with no theme or dark-mode customization. Replace the missing and outdated desktop screenshots, align them with the homepage screenshots, and account for the widgets requiring manual capture.

Written by the indexing model from the issue text.