Docs: Nexus Proof Compression - Phase 2

[danielmarinq]

A full description of the Nexus Phase 2 Nova Proof compression mechanism

[tess-eract]

@guruvamsi-policharla and I discussed this part of the design in some detail yesterday.

this mechanism is under active development and will likely look substantially different than the current partial implementation, and is not currently ready for a publicly visible PR. i think the discussion in the blog post frames the design questions well and gives important context for it. i don't think there's much else we can explain at this stage.