Add document-format RFC draft (#4264)

* Add document-format RFC draft

* Align document-format RFC with graph-storage implementation

* Update writing style

* Recharacterize resource source chain as LWW-element-set, not add-wins OR-set

* Add Rev identity rationale framed against Git, plus patch-id future item

* Correct RFC inaccuracies found in implementation audit

* Mark unimplemented retirement coarsening and leader election as future; fix resurrection mechanism

* Add hot-log to the on-disk payload list in the architecture diagram

Author: TrueDoctor