Viewport documentation needs updates/clarifying

**Your Godot version:**
4.x

**Issue description:**
1) Scale and stretching aren't clarified enough,
2) First time users reading this will assume "I am not using any viewports yet, I don't need to click the link" even though stretch modes apply to the root viewport too

The Viewport/SubViewport API has a similar issue (https://docs.godotengine.org/en/latest/classes/class_subviewport.html#class-subviewport). Even now as I write it I'm not 100% clear whether size is the viewport size or design size? What happens if someone tries to set/get this size for the root viewport?

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/tutorials/rendering/viewports.html