docs.voltha.org::HOWTO edits and WARNING cleanups.

Makefile
--------
  o Added convenience target 'make warnings' to help search for problems.

config.py
---------
  o Add more exclusions for .venv/ so sphinx will stop checking sources.

howto/edit_voltha_docs.rts
howto/docs.voltha.org/index.rst
howto/docs.voltha.org/quickstart.rst
howto/docs.voltha.org/edit_voltha_docs.rst
------------------------------------------
  o Added a quickstart guide for editing repo:voltha-docs

Misc
----
  o Fixed several WARNINGS
  o Add missing documents in index.rst

Change-Id: I0b3d01cc4116d40bd84db49649669253d6135546
diff --git a/Makefile b/Makefile
index 7d76ccb..3de80c0 100644
--- a/Makefile
+++ b/Makefile
@@ -191,6 +191,22 @@
  && $(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
 ## -----------------------------------------------------------------------
+## Intent: Display WARNINGS buried in sphinx output
+## -----------------------------------------------------------------------
+warnings-log := warnings.log
+warnings:
+	$(MAKE) html 2>&1 \
+	    | sed -e 's@\([Ww][Aa][Rr][Nn][Ii][Nn][Gg]\)@\n\1@g' \
+	    > "$(warnings-log)"
+	less --ignore-case --hilite-search --pattern='/warning' $(warnings-log)
+
+clean::
+	$(RM) $(warnings-log)
+
+help ::
+	@echo "  warnings              Display WARNING strings buried in sphinx output"
+
+## -----------------------------------------------------------------------
 ## Intent: Display make hel footer
 ## -----------------------------------------------------------------------
 include $(MAKEDIR)/help/trailer.mk