Document detailed attested build environment verification flow

[marcelamelara]

The current build environment track levels spec only describes verification at a very high level. As part of the track, we need to write up a dedicated verification document that lays out all of the attestations/data needed to verify the integrity of a build environment, the policies used to verify against, attestation/policy storage, and responsible actors for each aspect.

This was also raised in
#1115 (comment)
#1115 (comment)

[marcelamelara]

Also related comment: https://github.com/slsa-framework/slsa/pull/1115/files#r1790435248

[tiziano88]

cc @thmsbinder @ipetr0v @conradgrobler perhaps some of the oak building blocks may be useful here