Change standard material defaults and update docs (bevyengine#7664)

# Objective

Standard material defaults are currently strange, and the docs are wrong re: metallic.

## Solution

Change the defaults to be similar to [Godot](godotengine/godot#62756).

---

## Changelog

#### Changed

- `StandardMaterial` now defaults to a dielectric material (0.0 `metallic`) with 0.5 `perceptual_roughness`.

## Migration Guide

`StandardMaterial`'s default have now changed to be a fully dielectric material with medium roughness. If you want to use the old defaults, you can set  `perceptual_roughness = 0.089` and `metallic = 0.01` (though metallic should generally only be set to 0.0 or 1.0).

crates/bevy_pbr/src/pbr_material.rs

@@ -78,22 +78,27 @@ pub struct StandardMaterial {
 
     /// Linear perceptual roughness, clamped to `[0.089, 1.0]` in the shader.
     ///
-    /// Defaults to minimum of `0.089`.
+    /// Defaults to `0.5`.
     ///
     /// Low values result in a "glossy" material with specular highlights,
     /// while values close to `1` result in rough materials.
     ///
     /// If used together with a roughness/metallic texture, this is factored into the final base
     /// color as `roughness * roughness_texture_value`.
+    ///
+    /// 0.089 is the minimum floating point value that won't be rounded down to 0 in the
+    /// calculations used.
+    //
+    // Technically for 32-bit floats, 0.045 could be used.
+    // See <https://google.github.io/filament/Filament.html#materialsystem/parameterization/>
     pub perceptual_roughness: f32,
 
-    /// How "metallic" the material appears, within `[0.0, 1.0]`,
-    /// going from dielectric to pure metallic.
+    /// How "metallic" the material appears, within `[0.0, 1.0]`.
     ///
-    /// Defaults to `0.01`.
+    /// This should be set to 0.0 for dielectric materials or 1.0 for metallic materials.
+    /// For a hybrid surface such as corroded metal, you may need to use in-between values.
     ///
-    /// The closer to `1` the value, the more the material will
-    /// reflect light like a metal such as steel or gold.
+    /// Defaults to `0.00`, for dielectric.
     ///
     /// If used together with a roughness/metallic texture, this is factored into the final base
     /// color as `metallic * metallic_texture_value`.
@@ -236,19 +241,16 @@ pub struct StandardMaterial {
 impl Default for StandardMaterial {
     fn default() -> Self {
         StandardMaterial {
+            // White because it gets multiplied with texture values if someone uses
+            // a texture.
             base_color: Color::rgb(1.0, 1.0, 1.0),
             base_color_texture: None,
             emissive: Color::BLACK,
             emissive_texture: None,
-            // This is the minimum the roughness is clamped to in shader code
-            // See <https://google.github.io/filament/Filament.html#materialsystem/parameterization/>
-            // It's the minimum floating point value that won't be rounded down to 0 in the
-            // calculations used. Although technically for 32-bit floats, 0.045 could be
-            // used.
-            perceptual_roughness: 0.089,
-            // Few materials are purely dielectric or metallic
-            // This is just a default for mostly-dielectric
-            metallic: 0.01,
+            // Matches Blender's default roughness.
+            perceptual_roughness: 0.5,
+            // Metallic should generally be set to 0.0 or 1.0.
+            metallic: 0.0,
             metallic_roughness_texture: None,
             // Minimum real-world reflectance is 2%, most materials between 2-5%
             // Expressed in a linear scale and equivalent to 4% reflectance see