Rework of key encryption mode section

[laurencelundblade]

This is NOT ready to merge, but I'm making it available sooner rather than later.

I'm having trouble with the document make system, so this may break the CI.

[hannestschofenig]

Hi Laurence,

I think it is the wrong time to re-write the section again. If you have some specific comments, please focus on those. This document has been in development for a long, long time already. Additionally, we have synced the content with the JOSE HPKE work as well.

[selfissued]

I agree with Hannes. This is far more invasive than is called for at this stage in the draft (WGLC).

Please close this PR and if you want, replace it with a different one that fixes specific local problems in the exposition.

[laurencelundblade]

Understood that we need to work on this one specific problem at a time. I've opened some issues to describe the problems.

I had to do the rewrite to get my thinking clear. Sorry for the trouble caused by dumping it in one PR.