[Docs] Add example to demonstrate how to use skinning and morph with custom vertex shaders

## How can Bevy's documentation be improved?
I was writing a custom shader for an animated model. It continued to throw errors about the VERTEX[5] input buffer. After intense searching I discovered a vertex shader file in a community project, which offers some help:
After reading it I went to realize that bevy attempts to store the data about animation at position 5, which is not defined in my custom vertex shader.
An example about how to leverage animation data and morph target data in custom vertex shaders should be added. It should be noted that the skinning system uses position 5 and 6 as the joint buffer. The example should further demonstrate the way to handle skinning and morphing in vertex shader.