Update `bb` installation instructions

The instructions on this page are out of date: https://github.com/AztecProtocol/aztec-packages/tree/master/barretenberg/cpp/src/barretenberg/bb#installation, should recommend using `bbup.dev`

related https://github.com/AztecProtocol/aztec-packages/issues/9256