editorial: index.html use TR changes link, link to report source

update link to changes between 2e and 3e to use TR rather than githubio.

add a brief sentence / link to make the source of this report itself more easily discoverable on GitHub

Author: tantek

Implementation_Report_3e/index.html

@@ -69,13 +69,17 @@ <h2>Introduction</h2>
                 </p>
 
                 <p>
-                    The <a href="https://w3c.github.io/PNG-spec/#changes-20031110">changes 
+                    The <a href="https://www.w3.org/TR/png-3/#changes-20031110">changes 
                     in PNG Third Edition</a>,
                     compared to the Second Edition
                     are divided into new (or newly-standardized) features,
                     substantive changes to existing features,
                     and many editorial corrections and clarifications.
                 </p>
+		<p>
+                    This implementation report is on 
+                    <a href="https://github.com/w3c/png/tree/main/Implementation_Report_3e">GitHub</a>.
+		</p>
             </section>
 
             <section id="apng">