"Using a Viewport as a texture" guide out of date for version 4.1.1

[TomSelleck101]

Your Godot version: v4.1.1.stable.official [bd6af8e0e]

Issue description: Guide references several things which don't exist in this version.

1. Cannot create a "Viewport" as class is abstract. Needs to be updated to use SubViewport.
2. No option to disable HDR
3. Unable to set anchors as guide shows, margin option no longer available
4. Following guide up to and including "Applying the texture" results in a grey texture without any gradient

URL to the documentation page: https://docs.godotengine.org/en/stable/tutorials/shaders/using_viewport_as_texture.html