doc: Document binary blobs

As approved by the TSC on February 9th, 2022:
https://docs.google.com/document/d/1_sFM1BTLHrwRA9pbNoD7XPHE3OuqXT0P04AlAS2dvTk/edit#heading=h.y6xurk7xxg5a

Original document that was voted on:
https://docs.google.com/document/d/1heqcv7dzGvM5rA9xpTMW3kyKJLpZsl2Gjsmr5eqBje8/edit#



This initial patch documents most of the agreed upon decisions, with
additional information and infrastructure to come next.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>