Create reference doc for useful bb commands

[jzaki]

Context: nargo wrapped some backend commands, now decoupled and best/only accessed via the zk backend directly.
Eg nargo info used to show gate count, now must be accessed directly via the backend in use, eg bb gates command

[Savio-Sou]

Thanks @jzaki!

Since it's specific to Barretenberg, might be a good idea to document in https://github.com/AztecProtocol/aztec-packages/blob/master/barretenberg/README.md and link to it as reference in the Noir docs.

[jzaki]

BB in Aztec repo