[docs] Introduce Noir in the Aztec docs

[critesjosh]

There is currently no specific introduction to Noir in the Aztec docs, but Noir is an integral component of writing Aztec contracts. We should not assume visitors to the Aztec docs have knowledge of Noir.

Perhaps a section on Noir should be added to the Important Concepts page with detail about how Aztec actually uses noir (e.g. for writing protocol circuits (include links to protocol specs), and aztec.nr). It would be good to explain a bit about how aztec.nr leverages features of Noir to build a complete smart contract programming language (e.g. traits, generics, meta-programming) as well as the fact that any libraries written for Noir can be used in Aztec contracts.