Update RTD links from latest to stable, rather

The current stable version of the docs should be preferred over the
unreleased docs.

Author: PiDelport

index.html

@@ -83,13 +83,13 @@ <h3><a href=
       <p>Our documentation is <a href=
       "https://pillow.readthedocs.io/">hosted on readthedocs.io</a>
       and includes <a href=
-      "https://pillow.readthedocs.io/en/latest/installation.html">installation
+      "https://pillow.readthedocs.io/en/stable/installation.html">installation
       instructions</a>, <a href=
-      "https://pillow.readthedocs.io/en/latest/handbook/index.html">handbook</a>,
+      "https://pillow.readthedocs.io/en/stable/handbook/index.html">handbook</a>,
       <a href=
-      "https://pillow.readthedocs.io/en/latest/reference/index.html">API
+      "https://pillow.readthedocs.io/en/stable/reference/index.html">API
       reference</a>, <a href=
-      "https://pillow.readthedocs.io/en/latest/releasenotes/index.html">release
+      "https://pillow.readthedocs.io/en/stable/releasenotes/index.html">release
       notes</a> and more.</p>
 
       <h3>Discussion</h3>