Add template images to the Cubemap class documentation

This also adds an example sky shader for using a cubemap as a skybox.
This solution should be preferred to converting the cubemap texture
to a panorama sky, as that method implies a loss in quality compared
to the source cubemap.

doc/classes/Cubemap.xml

@@ -6,8 +6,27 @@
 	<description>
 		A cubemap is made of 6 textures organized in layers. They are typically used for faking reflections in 3D rendering (see [ReflectionProbe]). It can be used to make an object look as if it's reflecting its surroundings. This usually delivers much better performance than other reflection methods.
 		This resource is typically used as a uniform in custom shaders. Few core Godot methods make use of [Cubemap] resources.
-		To create such a texture file yourself, reimport your image files using the Godot Editor import presets.
+		To create such a texture file yourself, reimport your image files using the Godot Editor import presets. The expected image order is X+, X-, Y+, Y-, Z+, Z- (in Godot's coordinate system, so Y+ is "up" and Z- is "forward"). You can use one of the following templates as a base:
-		[b]Note:[/b] Godot doesn't support using cubemaps in a [PanoramaSkyMaterial]. You can use [url=https://danilw.github.io/GLSL-howto/cubemap_to_panorama_js/cubemap_to_panorama.html]this tool[/url] to convert a cubemap to an equirectangular sky map.
+		- [url=https://raw.githubusercontent.com/godotengine/godot-docs/master/img/cubemap_template_2x3.webp]2×3 cubemap template (default layout option)[/url]
+		- [url=https://raw.githubusercontent.com/godotengine/godot-docs/master/img/cubemap_template_3x2.webp]3×2 cubemap template[/url]
+		- [url=https://raw.githubusercontent.com/godotengine/godot-docs/master/img/cubemap_template_1x6.webp]1×6 cubemap template[/url]
+		- [url=https://raw.githubusercontent.com/godotengine/godot-docs/master/img/cubemap_template_6x1.webp]6×1 cubemap template[/url]
+		[b]Note:[/b] Godot doesn't support using cubemaps in a [PanoramaSkyMaterial]. To use a cubemap as a skybox, convert the default [PanoramaSkyMaterial] to a [ShaderMaterial] using the [b]Convert to ShaderMaterial[/b] resource dropdown option, then replace its code with the following:
+		[codeblock lang=text]
+		shader_type sky;
+
+		uniform samplerCube source_panorama : filter_linear, source_color, hint_default_black;
+		uniform float exposure : hint_range(0, 128) = 1.0;
+
+		void sky() {
+		    // If importing a cubemap from another engine, you may need to flip one of the `EYEDIR` components below
+		    // by replacing it with `-EYEDIR`.
+		    vec3 eyedir = vec3(EYEDIR.x, EYEDIR.y, EYEDIR.z);
+		    COLOR = texture(source_panorama, eyedir).rgb * exposure;
+		}
+		[/codeblock]
+		After replacing the shader code and saving, specify the imported Cubemap resource in the Shader Parameters section of the ShaderMaterial in the inspector.
+		Alternatively, you can use [url=https://danilw.github.io/GLSL-howto/cubemap_to_panorama_js/cubemap_to_panorama.html]this tool[/url] to convert a cubemap to an equirectangular sky map and use [PanoramaSkyMaterial] as usual.
 	</description>
 	<tutorials>
 	</tutorials>