a minimal core intent proposal

I wrote a proposal <https://w3c.github.io/mathml-docs/minimal-intent-core>
for simplifying what goes into intent core. It ended up being sort of an
"AT requirements" document for core. If I extend it a little further to
include what AT should do with "intent" (currently just presumed everyone
knows), it would be the basis for an actual AT requirements document (or
appendix). It also serves to let authors/authoring software know what they
can count on as default behavior by AT.

The proposal contains some open questions, but I believe it is fleshed out
enough that it is understandable and actionable (let's do this/don't do
this). It extends what I put in Deyan's intent spreadsheet and also has
explanations. It will be the basis for the third agenda item on Thursday.

    Neil