Combine redundant sections on setting materials and link to new section

**Your Godot version:**
4.4
**Issue description:**
Godot has several places where materials can be assigned:
- On the Mesh resource's [material](https://docs.godotengine.org/en/stable/classes/class_mesh.html#class-mesh-method-surface-get-material)
- On the MeshInstance3D's [surface material override](https://docs.godotengine.org/en/stable/classes/class_meshinstance3d.html#class-meshinstance3d-method-get-surface-override-material)
- On the GeometryInstance3D's [material override](https://docs.godotengine.org/en/stable/classes/class_geometryinstance3d.html#class-geometryinstance3d-property-material-override) or (material overlay)

This is a frequent cause of confusion. It *is* explained in two places:
- In the [first 3d shader](https://docs.godotengine.org/en/stable/tutorials/shaders/your_first_shader/your_first_3d_shader.html#where-to-assign-my-material) tutorial
- In the [Standard Material 3D introduction](https://docs.godotengine.org/en/stable/tutorials/3d/standard_material_3d.html#introduction)

And referenced in at least one other, in the [GridMap tutorial page](https://docs.godotengine.org/en/latest/tutorials/3d/using_gridmaps.html#materials)

**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