Add initial 2.1 design doc

This document contains a skeleton for the 2.1 design process.
For now it just has introductory paragraphs and a structure for the
various areas' design, but some sections still don't have a text, as
we're still in the early design phases.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>