Initial revisions to b2d doc, new screenshots.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)

Final revisions to B2D doc.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #7169 from fredlf/update_osx-install

Initial revisions to b2d doc, new screenshots.