Add 3->4 upgrading information about NDC, light(), and reverse-z

[tetrapod00]

Resolves godotengine/godot#96704 by changing the docs.

• Adds information about normalized device coordinates changing from 3.x/Compatibility to Forward+/Mobile.
• Adds a note that shaders with custom lighting will need changes.
• Links to reverse-z changes. As more and more 4.x versions are released, reverse-z will become the default, and most users finding this page will need to apply reverse-z changes if their shader needs it. We should not expect users to visit each minor version conversion page in sequence in order to find this information.

---

Summary of the original issue: a simplified version of the shader in https://github.com/paddy-exe/Godot-RealTimeCaustics was converted automatically from 3 to 4, then all the manual fixes mentioned here were applied. But the shader still has no visible output.
Two additional changes had to be made so the shader works in 4.3 Forward+:

• Convert the NDC reconstruction from vec3 ndc = vec3(SCREEN_UV, depth) * 2.0 - 1.0; to vec3 ndc = vec3(SCREEN_UV* 2.0 - 1.0, depth);.
• Change ALBEDO = vec3(0.0); to ALBEDO = vec3(1.0);. The shader is doing additive lighting using the light() function, and requires a change to ALBEDO to look similar to 3.x.

These kinds of changes cannot be supported by an automatic conversion tool, and they can't even be exhaustively listed. We can list common cases, and clarify the warning that advanced shaders will need more manual work.

I didn't actually use 3.x, so my knowledge of this is only from converting shaders.

[clayjohn]

Flarkk's recommendations are good. Everything else looks good, so this can be merged once you make those changes

[tetrapod00]

I think that's everything. Thank you for the reviews!

[clayjohn]

Thank you!

[mhilbrunner]

@tetrapod00 Thank you! Merged.

[mhilbrunner]

Cherrypicked to 4.3 in #10346.