doc: add tool for verifying installed doc tools

Added a python script that displays versions of doc building tools
(e.g., doxygen, breathe, sphinx) to help verify doc building environment
is properly set up (can help doc building problem diagnostic, as shown
in issue #1333

Update doc build documentation to reference this script.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/scripts/show-versions.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+#
+# Copyright (c) 2018, Intel Corporation
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+# Show installed versions of doc building tools
+
+import subprocess
+
+print ("breathe          version: " +
+          __import__("breathe").__version__ + "\n" +
+       "docutils         version: " +
+          __import__("docutils").__version__ + "\n" +
+       "sphinx           version: " +
+          __import__("sphinx").__version__ + "\n" +
+       "sphinx_rtd_theme version: " +
+          __import__("sphinx_rtd_theme").__version__ + "\n" +
+       "doxygen          version: " +
+          subprocess.check_output(["doxygen", "-v"]).decode("utf-8"))