Add TOC header in RTD index page

Refs NOAA-EMC#2395

docs/source/index.rst

@@ -27,6 +27,10 @@ General updates: NOAA employees and affiliates can join the gfs-announce distrib
 
 GitHub updates: Users should adjust their "Watch" settings for this repo so they receive notifications as they'd like to. Find the "Watch" or "Unwatch" button towards the top right of the `authoritative global-workflow repository page <https://github.com/NOAA-EMC/global-workflow>`_ and click it to adjust how you watch the repo.
 
+=================
+Table of Contents
+=================
+
 .. toctree:: 
    :numbered:
    :maxdepth: 3