website: add link to the release archive

We have them, let them be reachable.

[Peter: use relative URL like elsewhere]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/website/download.html

@@ -47,6 +47,9 @@ want to follow development, but cannot or do not wish to use Git.
 	</li>
 </ul>
 
+<p>
+Older versions can be downloaded from <a href="/downloads/">the release archive</a>.
+</p>
 
 <h2>Repository</h2>