editorial: adjust contributors section (#211)

Drops the previously active section and adds a new GH contributors section, matching other ARIA specs.

See w3c/aria-common#103 for more background.

index.html

@@ -9502,8 +9502,10 @@ <h2>Substantive changes since the <a href="https://www.w3.org/TR/core-aam-1.1/">
 	<section class="appendix informative section" id="acknowledgements">
 		<h3>Acknowledgments</h3>
 		<p>The following people contributed to the development of this document.</p>
+		<ul id="gh-contributors">
+			<!-- list of contributors will appear here, along with link to their GitHub profiles -->
+		</ul>
 		<div data-include="common/acknowledgements/aria-wg-active.html" data-include-replace="true"></div>
-		<div data-include="common/acknowledgements/aria-contributors.html" data-include-replace="true"></div>
 		<div data-include="common/acknowledgements/funders.html" data-include-replace="true"></div>
 	</section>
 </body>