Clarify purpose of shader_instancing example (#15456)

# Objective

- The shader_instancing example can be misleading since it doesn't
explain that bevy has built in automatic instancing.

## Solution

- Explain that bevy has built in instancing and that this example is for
advanced users.
- Add a new automatic_instancing example that shows how to use the built
in automatic instancing
- Rename the shader_instancing example to custom_shader_instancing to
highlight that this is a more advanced implementation

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Co-authored-by: JMS55 <47158642+JMS55@users.noreply.github.com>