Update the specification with the design

[ch1bo]

Why

Having a concise write-up of the how to deal with incremental decommits will inform implementation of the feature and make us more confident on whether all things have been considered.

What

• Update the LaTeX document with the design from #1057 in a dedicated PR.
  • Using this comment https://github.com/input-output-hk/hydra/issues/1057#issuecomment-2042769014 as input
  • Mark up all differences to the current released protocol (in red)
• Collect a list of where the implementation needs to change to cover the spec changes.
  • Compared to the current released protocol

How

• Sebastian does a write-up (others might join)
• Everyone reviews it
• One Q&A session