[TF-M] Move dual-cpu design document into code repo

Hi all,

We are moving dual-cpu design documents from trustedfirmware.org wiki pages into TF-M code repo. We convert the documents to rst format, fix some typos, update them to align with latest implementation.

The patches have been reviewed for several rounds in https://review.trustedfirmware.org/q/topic:%22dualcpu-docs%22+(status:open%2...). The documents patches will be merged soon by this week.

Please comment on the patches if there is any serious issue in the design. Any suggestion or improvement is still welcome after the patches are merged!

Best regards, Hu Ziji