Add In Memoriam text for @gkellogg

Author: anatoly-scherbakov

spec/index.html

@@ -2756,5 +2756,29 @@ <h3>Fragment identifiers</h3>
         </div>
       </section>
     </section>
+
+    <section id="ack" class="appendix informative">
+      <h2>Acknowledgements</h2>
+
+      <h3>In Memoriam</h3>
+      <p>Gregg Kellogg was a central figure in the story of JSON-LD and YAML-LD.
+      He worked tirelessly on JSON-LD and many other specifications for more than a decade,
+      right up until his death on September 6th, 2025. Gregg's passion for solving
+      hard problems with finesse was only exceeded by his willingness to collaborate
+      with and encourage others in reaching that destination. The JSON-LD and
+      broader Linked Data communities will be forever grateful for Gregg's
+      consistent, careful, and kindly delivered contributions during their most
+      formative years.</p>
+
+      <h3>Contributions</h3>
+      <p>
+          The editors would like to especially thank the following individuals for making significant
+          contributions to the authoring and editing of this specification:
+      </p>
+
+      <p>
+        <em>To be added.</em>
+      </p>
+    </section>
   </body>
 </html>