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/conf.py b/conf.py
index cfd7dd4..9290b6f 100644
--- a/conf.py
+++ b/conf.py
@@ -88,7 +88,9 @@
 ]
 
 # don't include tags
-smv_tag_whitelist = None
+# smv_tag_whitelist = None
+# smv_tag_whitelist = disabled^ # Was = None
+smv_tag_whitelist = 'disabled_a^' # Was = None
 
 # inlcude only the branches matching master and voltha-*
 smv_branch_whitelist = r'^(master|voltha-.*)$'
@@ -138,9 +140,12 @@
         'CODE_OF_CONDUCT.md',
         '*/CODE_OF_CONDUCT.md',
 # Ignore all:
+        '__pycache__',
         '**/__pycache__', # generated - faster traversal
+        '.venv',
+        '**/.venv',       # external virtual env
 # Revisit: one directory pattern is sufficient.
-        '**/doc_venv',
+        '**/doc_venv', 
         '**/venv_docs',
         '**/virtualenv',
         '**/vst_venv',