Document mapping between godot types and godot shader language types

**Your Godot version:**
4.6

**Issue description:**

`ShaderMaterial` has two public methods `get_shader_parameter()` and `set_shader_parameter()` that each return or accept values of type `Variant`.

These function act as an interface between the general purpose Godot scripting languages and godot shader language. These two domains have different types and it is unclear which godot types map to which godot shader language types.

A table documenting these type mappings would help the docs stand alone. As it is I am left inferring type compatibility by reviewing the Editor UI e.g. these can be assigned to a `sampler2D` via the GUI, but are they all compatible with the getter/setter methods?

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/classes/class_shadermaterial.html
https://docs.godotengine.org/en/stable/tutorials/shaders/index.html