Pydoc: Move builtin GPU shader information to code

This fixes a compile warning for sphinx.
This commit includes some reformating of the information.

doc/python_api/examples/gpu.shader.py

@@ -1,36 +0,0 @@
-"""
-Built-in shaders
-++++++++++++++++
-
-All built-in shaders have the ``mat4 ModelViewProjectionMatrix`` uniform.
-The value of it can only be modified using the :class:`gpu.matrix` module.
-
-2D_UNIFORM_COLOR:
-   attributes: vec3 pos
-   uniforms: vec4 color
-
-2D_FLAT_COLOR:
-   attributes: vec3 pos, vec4 color
-   uniforms: -
-
-2D_SMOOTH_COLOR:
-   attributes: vec3 pos, vec4 color
-   uniforms: -
-
-2D_IMAGE:
-   attributes: vec3 pos, vec2 texCoord
-   uniforms: sampler2D image
-
-3D_UNIFORM_COLOR:
-   attributes: vec3 pos
-   uniforms: vec4 color
-
-3D_FLAT_COLOR:
-   attributes: vec3 pos, vec4 color
-   uniforms: -
-
-3D_SMOOTH_COLOR:
-   attributes: vec3 pos, vec4 color
-   uniforms: -
-
-"""