docs: Updated Google Play distribute docs

Updating docs based on source material from Dom. I've had to tweak
the image formatting & arrangement to work with DAC styles, but
the text is pretty much unchanged.

Per the bug, this CL just has the "P0: Ready To Go" docs.

See first comment for doc stage location.

bug: 25596006
Change-Id: I508b0bb92a807ece8b9b0bf0ee7b49e0bbbc2432