[ZIP 374] PCZT v1 format

[str4d]

TODO: Finish writing the initial draft by filling out the TODOs.

Fixes #693

[str4d]

Assigned ZIP 374 in ZIP Sync today with @daira, @nuttycom, @SamHSmith.

[str4d]

Decision in ZIP Sync today with @daira @arya2 @nuttycom: from a process perspective, we do allow Draft ZIPs (i.e. documents with assigned ZIP numbers that are in Draft status) to be incomplete and have potentially significant TODOs; this leans into the "work-in-progress" wording in the README and on zips.z.cash. We already track these in a separate category in those from the Released ZIPs to avoid confusion, and the transition between those two categories is the point at which the TODOs must have all been completed.

Following this, I will update this PR to number the draft appropriately, and then merge it so we can get a rendered reference up to the parts of the PCZT ZIP that have been written.