Move doc generation back into main workspace

[brandjon]

In other words, undo f9dc57111332d04ea5de63dacfb84013ebff9188.

According to that commit's description, the justification for having two workspaces was that Bazel did not well support having both Python 2 and 3 in the same build, and Skydoc did not support Python 3. The issue of two versions in the same build should be addressed once the incompatible flags in Bazel 0.23 are flipped on by default.