Update documentation for v0.6.0 release

[adrhill]

Currently, the following things can be improved or are missing documentation:

• input augmentations: NoiseAugmentation, InterpolationAugmentation
• usage of LayerMap and show_layer_indices introduced in Support nested indexing in composite primitive LayerMap #131
• LRP keyword flatten and performance benefits
• LRP model canonization
• Update section "How it works internally" for Support nested Flux Chains in LRP #119
• Update "Model checks for humans" for Support nested Flux Chains in LRP #119
• Use DocumenterCitations.jl
• Document use of LoopVectorization.jl and CUDA.jl  #64