Debian/Ubuntu: Cleanup and update index pages

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

docs/Getting Started/Ubuntu/index.rst

@@ -1,8 +1,20 @@
 Ubuntu
 ======
 
-ZFS packages are `provided by the
-distribution <https://wiki.ubuntu.com/Kernel/Reference/ZFS>`__.
+.. contents:: Table of Contents
+  :local:
+
+Installation
+------------
+
+If you want to use ZFS as your root filesystem, see the `Root on ZFS`_
+links below instead.
+
+Since Ubuntu 16.04, ZFS is included in the default Linux kernel packages.  To
+install the ZFS utilities::
+
+  apt update
+  apt install zfsutils-linux
 
 Root on ZFS
 -----------
@@ -10,4 +22,4 @@ Root on ZFS
    :maxdepth: 1
    :glob:
 
-   *
+   *