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/howto/release/index.rst b/howto/release/index.rst
index 6dfa10f..ce276ee 100644
--- a/howto/release/index.rst
+++ b/howto/release/index.rst
@@ -14,6 +14,8 @@
    voltha-docs/index
    post-release/index
 
+   charts/index
+
    voltctl/index
    voltctl/install
    voltctl/release