Document integer scaling support in Multiple resolutions

[Calinou]

This also adds more recommendations for base viewport sizes in pixel art games.

Don't cherry-pick, as this feature is only in 4.2.

• See Add content scale stretch modes, implement integer scaling godot#75784 and Add integer scaling to Multiple resolutions demo godot-demo-projects#985.

[skyace65]

This will supersede #8231.

[mhilbrunner]

Thanks! Merged.