ADR for NOFO download

[mxk0]

Summary

Includes:

• Near-term solution to deliver download for 5 SimplerNOFOs
• Prod-ready solution for NOFO download
• Definition of the MVP data model to handle multiple documents per opportunity

Note: #2123 and #2216 have details/thoughts on the data model. Also this comment.

Note: product requirements gathering for document UX will eventually happen in #2437. As of now it doesn't seem like that's a blocker to this ticket happening.

Acceptance criteria

• [ ] ADR reviewed and finalized

[mxk0]

@chouinar just a heads up, we added the question you raised about document hierarchy/data model to this ADR.