Annotate design documents with the "state"

We are accumulating design documents under https://github.com/dandi/dandi-archive/tree/master/doc/design which is great. But as time goes it becomes unclear on what is the destiny of each design document -- was it just proposed, being implemented superseded, etc. I propose to add some header structure, could be just a markdown list of keys like

- State: proposed (other values: work-in-progress, partially-implemented,implemented, abandoned, superseded; could be mixed, e.g., "implemented,superseded")
- Released-In: ``
- Superseded-By: ``
- Superseded-In: ``
- Implemented-In: ``

WDYT? I know that it would be a bit more of work, but would bring clarity since ATM not clear in what state we are in for each of those designed docs.

Review the design documents under doc/design and the proposed State, Released-In, Superseded-By, Superseded-In, and Implemented-In fields. First determine the agreed metadata structure and valid values from the issue discussion, then annotate the relevant documents consistently. Done means each design document clearly records its implementation status and related version, document, or pull-request references where applicable.

Written by the indexing model from the issue text.