Metadata: Use 'environment' property

### Problem

AppCenter (the desktop app) supports filtering screenshot using `environment` property of `screenshot` tag, but the documentation does not use it, resulting the developers may not notice this great feature.

https://docs.elementary.io/develop/writing-apps/our-first-app/metadata#screenshots

### Proposal

Write about use of `environment` property.

- Use `pantheon` for light style and `pantheon:dark` for dark style as its value

### Prior Art (Optional)

FYI: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-screenshots:~:text=present%2E-,Optionally