doc: add documentation for mailbox framework
doc: add documentation for mailbox framework

 Some explanations with examples of how to write to implement users
and providers of the mailbox framework.

Signed-off-by: Jassi Brar <191fbc354bb0d45dfb103e8f4f6583bfe2bb3ef7@linaro.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs