Version the rig documentation

[grayside]

With the forthcoming rig v2, we are deprecating/removing a couple things. We should be sure to mark a 1.x rev or branch of the documentation for anyone that gets stuck using rig 1.x for awhile for a project that does not port to things like the unison file sync.

[febbraro]

MkDocs does not appear to support this natively so we would likely need a more complicated build process to build into 2 different subdirs v1/ and current/