Fix link to Aaron Toponce's tutorial

It seems that Aaron Toponce's tutorial is no longer online. So let's link to the latest archived version on archive.org instead.

Technical details: While the domain pthree.org still resolves to a proper IP address, as well an IPv4 address, there appears to be no webserver behind it to respond, neither on port 443 nor on legacy port 80.

Author: vog

docs/Getting Started/index.rst

@@ -5,7 +5,7 @@ To get started with OpenZFS refer to the provided documentation for your
 distribution. It will cover the recommended installation method and any
 distribution specific information. First time OpenZFS users are
 encouraged to check out Aaron Toponce's `excellent
-documentation <https://pthree.org/2012/04/17/install-zfs-on-debian-gnulinux/>`__.
+documentation <https://web.archive.org/web/20230904234829/https://pthree.org/2012/04/17/install-zfs-on-debian-gnulinux/>`__.
 
 .. toctree::
    :maxdepth: 3