PEP 740: tweak JSON simple API prescriptions

[woodruffw]

Per discussion with @dstufft: this removes the embedded provenance objects from the simple API and replaces them with digest references, much like the simple index. This has the virtuous effect of reducing the amount of mostly chaff JSON that client API consumers will need to download.

The added Appendix 3 has further details, including a rationale and concrete numbers. These have also been shared in the discussion thread.

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