Find a good way to document proofs

As we add more and more proofs about the spec to the repository, we should think about a good way to document what we have proven.
This does not mean documenting every single lemma, but the headline theorems. E.g., in a latex companion. Or just a list or issue would already be a good start.

Start by surveying the repository's existing proofs and identifying the headline theorems, rather than individual lemmas. Done means choosing and documenting a maintainable format, such as a LaTeX companion or a concise theorem list, with the proven results represented clearly.

Written by the indexing model from the issue text.