The "Your first/second 3D shader" tutorials are outdated and provide incorrect results

**Your Godot version:**
v4.1.1

**Issue description:**
The tutorials on 3D shaders do not work properly and give unexpected results, especially when calculating normals

What I expected and what I recieved:

In general the entire tutorial is outdated and provides unexpected results in-engine and should be massively revised.

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/tutorials/shaders/your_first_shader/your_second_3d_shader.html

If you know how to fix the issue you are reporting please
consider opening a pull request. We provide a tutorial on
using git here: https://docs.godotengine.org/en/stable/community/contributing/pr_workflow.html,
writing documentation at https://docs.godotengine.org/en/stable/community/contributing/docs_writing_guidelines.html
and contributing to the class reference here: https://docs.godotengine.org/en/stable/community/contributing/updating_the_class_reference.html