Document how to read stored subscriptions for governance, metrics, etc

[turadg]

What is the Problem Being Solved?

Agoric/agoric-sdk#5678 (comment)

Description of the Design

It should be a documentation snippet.

Maybe also integrate with the ETL.

Security Considerations

Test Plan

[dckc]

maybe should move to the agoric/documentation repo

[dckc]

substantial overlap with

• Document off-chain data lookup (aka vstorage RPC) #758

[dckc]

done by the combination of

• Vstorage journey - 4daydapp #947
• UI tutorial #1072