Debian/Ubuntu: Add "Overview" header

This improves the table of contents.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Ubuntu/Ubuntu 16.04 Root on ZFS.rst

@@ -4,20 +4,23 @@ Ubuntu 16.04 Root on ZFS
 .. contents:: Table of Contents
    :local:
 
+Overview
+--------
+
 Newer release available
------------------------
+~~~~~~~~~~~~~~~~~~~~~~~
 
 -  See :doc:`Ubuntu 18.04 Root on ZFS <./Ubuntu 18.04 Root on ZFS>` for new installs.
 
 Caution
--------
+~~~~~~~
 
 -  This HOWTO uses a whole physical disk.
 -  Do not use these instructions for dual-booting.
 -  Backup your data. Any existing data will be lost.
 
 System Requirements
--------------------
+~~~~~~~~~~~~~~~~~~~
 
 -  `64-bit Ubuntu 16.04.5 ("Xenial") Desktop
    CD <http://releases.ubuntu.com/16.04/ubuntu-16.04.5-desktop-amd64.iso>`__
@@ -35,7 +38,7 @@ RAM <http://wiki.freebsd.org/ZFSTuningGuide#Deduplication>`__. Enabling
 deduplication is a permanent change that cannot be easily reverted.
 
 Support
--------
+~~~~~~~
 
 If you need help, reach out to the community using the `zfs-discuss
 mailing list <https://github.com/zfsonlinux/zfs/wiki/Mailing-Lists>`__
@@ -45,7 +48,7 @@ a new issue <https://github.com/zfsonlinux/zfs/issues/new>`__ and
 mention @rlaager.
 
 Contributing
-------------
+~~~~~~~~~~~~
 
 1) Fork and clone: https://github.com/openzfs/openzfs-docs
 
@@ -75,7 +78,7 @@ Contributing
    Mention @rlaager.
 
 Encryption
-----------
+~~~~~~~~~~
 
 This guide supports the three different Ubuntu encryption options:
 unencrypted, LUKS (full-disk encryption), and eCryptfs (home directory