MathML Intent explainer

This week we will have at least one AT vendor join the meeting. As
discussed last week, we need to have something relatively short and simple
for them to read to get some background on "intent".

I have created a first draft
<https://w3c.github.io/mathml-docs/intent-explainer/>. Please comment on it
at this issue <https://github.com/w3c/mathml/issues/461> by EOD Monday as I
need to send it off so our guests have some time to read it and think about
it.

    Neil