Update lint target documentation.

howto/code/index.rst
--------------------
  o Change table-of-contents depth from 2 to 1.
  o {topic}/index.rst can render sub-index content.

howto/code/linting.rst
----------------------
  o Document doc8 lint to reflect latest targets in repo:onf-make.
  o Update header tokens to render w/o the need for (TOC) table-of-contents formatting.

See Also
--------
  o https://docs.voltha.org/master/howto/
  o https://docs.voltha.org/master/howto/code/linting.html

Change-Id: I684837c457c42c184620b87ad639c47f43061189
diff --git a/howto/code/index.rst b/howto/code/index.rst
index 3041736..9152cec 100644
--- a/howto/code/index.rst
+++ b/howto/code/index.rst
@@ -2,7 +2,7 @@
 =========================
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 1
 
    commit-message
    job-failures