docs backports

Hey folks, now that Pacific is out I wanted to bring up docs backports. Today, docs.ceph.com shows master by default, with an appropriate warning at the top that it represents a development version. Since the primary audience of the docs is users, not developers, I suggest that we switch the default branch to the latest stable, i.e. pacific, and apply the normal backport process to docs that are relevant to the latest stable release as well. To kickstart things, I'll prepare a backport of the existing doc changes since the pacific release. What do folks think? Josh