Add sphinxcontrib-versioning

- 'make versioned' target to build w/versions, and supporting 'make
  prep' target.
- Rework how checking out refs in repos are done in the makefile
- Change versioning method from VERSION file to conf.py
- use fork of sphinxconfig-versioning to deal with repos/ structure
- move OTHER_REPO_DOCS in Makefile so doc8 target requires that docs are
  checked out in repos/ .

Change-Id: I6f5d6eee324bbbe31b71e1af596f0d8feb8f8785
5 files changed