[Doc] Update LayerExtension examples to include return of a well defined shader module

[richard3d]

Link

https://deck.gl/docs/developer-guide/custom-layers/layer-extensions#example-layer-extension

Description

See #9122 for the origination of this suggestion. The LayerExtension dev-guide page does not provide very clear guidance on the shape of a shader module to be returned from getShaders() which can affect if an extension is properly merged with a layer's shader code as in the original problem. The examples are too minimal to provide insight as well. Another acceptable/appropriate location to mention this information might be in the writing-shaders.