doc: add Support Bundle design doc
This adds a new blueprint/design document for Support Bundles.
The first iteration of this document was discussed in https://docs.google.com/document/d/1mbnIVAdGa5n9twp4laOC0bj-1SSlrG7ahfdbIsdDsuE/edit#, and some useful feedback was provided on that document.
cc @vparsons @wchandler @katrinleinweber @joshlambert @bprescott_
Edited by Andrew Newdigate