Metadata: Use 'environment' property

[ryonakano]

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