Side nav docs: dark/alt variations should work without au-body

[sukhrajghuman]

we need to mention that the dark variations need to be wrapped in .au-body.au-body--dark in our docs

[sukhrajghuman]

It seems that the dark side-nav needs to be wrapped in a div with classes .au-body.au-body--dark

This is inconsistent with the default side navs and potentially confusing.