Add figure for build environment lifecycle showing metadata/attestation flows

[marcelamelara]

This figure needs to highlight different components that interact at each stage of the build env lifecycle, from build image creation to build dispatch. Something I'm still undecided about is whether this figure should also depict the attestation flow of the BuildEnv track.

[marcelamelara]

Upon further discussion, this diagram really ought to include the data flows during the lifecycle, i.e. the provenance generation/verification steps.