[ingestion/data-quality issue] Clarify in the README supported document types and formats

[mlieberman85]

In the README we have a list of "formats":

CycloneDX Dead Simple Signing Envelope Deps.dev API In-toto ITE6 OpenSSF Scorecard OSV SLSA SPDX

Two things:

1. Within GUAC's on internal code it refers to these as document types not formats
2. Format is used within the code to talk about the storage/transport format, e.g. XML, JSON, etc.

We should clarify this discrepancy as well as make clear what formats (XML, JSON, etc.) are supported by GUAC and by what document types.