Doc: Dataflow for submitting claims and evidence

[SteveLasker]

SCITT will typically be alongside an existing storage solution, providing claims, attestations and endorsements to some type of artifact.
This doc captures the data flow, what might be, or might not be transactional to support writing to a SCITT registry and some storage solution that captures the opaque evidence.

Updating to stub at: https://github.com/ietf-scitt/scitt-web/blob/main/scitt-storage-api-sequence.md

[SteveLasker]

@roywill was going to update this, or turn into a discussion for how we want to track id's for graphs that are put on the ledger for more complex build environments