Namada specs pre-mainnet order of operations

[cwgoes]

• Clean up the current specs, aiming for a reasonable level of completeness and consistency with the codebase. Sub-tasks:
  • Namada specs review #277
  • Review and update(?) Gas&Fee specs #201
  • Update replay protection specs #200
  • Document public API and encodings #256
• Figure out specs versioning and the relation to codebase update processes
• Merge other existing documents such as the shielded sync & state sync HackMDs

Separately, we should think about what to do with the docs. I think those are easier to keep separately for now, so they can remain here, but we should still think about versioning and maintenance processes.