[3.x] Shaders: Backport structs documentation

[dalexeev]

Structs were originally implemented for 4.0 and then backported to 3.4. But we probably forgot to backport the documentation too. A user in the local community pointed this out yesterday, structs work in 3.5 and 3.6, but are not mentioned in the documentation.

I'm not an expert in this area and don't know if there are any differences in the functionality between 3.x and 4.x, so please check if any corrections are needed in the documentation.

Also, as far as I understand, this documentation should be cherry-picked not only into 3.5 but also into 3.4. However, the label is missing and support for this version of the engine has been discontinued.

See:

• Implementation of 'struct' for shaders godot#35249
• Structs and fragment-to-light varyings for 3.4 godot#48075
• Added docs for shader struct #3178