Update "Your second 3D shader" for 4.5

[not-bottle]

Issue: #8077

Attempted to make the water shader in the "Your second 3d shader" tutorial more straightforward to reproduce in Godot 4 versions.

• Removed the suggestion to use the specular_toon render_mode. Since Godot 3 I think this has changed to affect the diffuse lighting and the highlights take on the albedo color, which I don't think produces the desired result in this shader anymore
• Attempted to clarify when/why the tutorial computes the normals manually, as this seemed to cause a lot of issues
• Explicitly stated points where mesh and noise parameters should be changed so results can match screenshots
• Added new images illustrating additional steps for noise parameter changes + screenshot updates

I left most of the original content as is but the section introducing the render modes is no longer really relevant to the tutorial. I left it as a note but wasn't sure whether to remove it or not. As is it's ~2000 words so it might need shortening.

The previous effect in Godot 3:

The modified effect in Godot 4.5 (no toon highlights unfortunately):