Combine redundant sections on setting materials and link to new section

[tetrapod00]

Your Godot version:
4.4
Issue description:
Godot has several places where materials can be assigned:

• On the Mesh resource's material
• On the MeshInstance3D's surface material override
• On the GeometryInstance3D's material override or (material overlay)

This is a frequent cause of confusion. It is explained in two places:

• In the first 3d shader tutorial
• In the Standard Material 3D introduction

And referenced in at least one other, in the GridMap tutorial page

Proposal:

• Unify the two existing descriptions into a single dedicated section. Specifically, expand the section in the standard material docs and give it a title. (Possibly there is a third, better place where this should be moved)
• Shorten the other redundant explanations, and link to the dedicated section on the topic.
• Look for more places where linking to the dedicated section would increase clarity.

URL to the documentation page (if already existing):
See above