Define schema and indexes as that are used within Sentinel

[placer14]

Model details are documented at https://lilium.sh/data/models/, but in order to be thoughtful about query performance it is helpful to know the indexes which exist over those models as defined in PL's schema. We could point people at https://github.com/filecoin-project/lily/blob/master/schemas/v1/base.go and the following migrations, but this is not documentation that's easy to find/access.

As a consumer of Sentinel schema, I would like documentation about the schema which models are persisted to in order to better understand how to consume data within that schema.