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
diff --git a/requirements.txt b/requirements.txt
index 4f5937c..d523bef 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -14,5 +14,9 @@
 sphinxcontrib-openapi~=0.5.0
 sphinxcontrib-seqdiag~=0.8.5
 sphinxcontrib-spelling~=4.2.1
+# using a fork as the main branch is broken in newer sphinx, per:
+# https://github.com/sphinx-contrib/sphinxcontrib-versioning/issues/58
+# also added functionality to support working with other repos
+git+https://github.com/zdw/sphinxcontrib-versioning@0998d870b2791eb6343c98e98d6680c1e727daee
 # sphinx-autoapi~=1.2.1
 # sphinxcontrib-golangdomain