add link to UI repository in footer text

Author: mojavelinux

src/partials/footer-content.hbs

@@ -14,7 +14,7 @@
   </div>
   <div class="footer-legal">
     <p>Copyright © {{{year}}} Dan Allen, Sarah White, and individual Asciidoctor contributors. Except where noted, the content is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) license.</p>
-    <p>The UI for this site is based on the default Antora UI and is licensed under the MPL-2.0 license. Several icons are imported from <a href="https://primer.style/octicons/" target="_blank" rel="noopener">Octicons</a> and are licensed under the MIT license.</p>
+    <p>The <a href="https://github.com/asciidoctor/asciidoctor-docs-ui" target="_blank" rel="noopener">UI for this site</a> is derived from the Antora default UI and is licensed under the MPL-2.0 license. Several icons are imported from <a href="https://primer.style/octicons/" target="_blank" rel="noopener">Octicons</a> and are licensed under the MIT license.</p>
     <p>AsciiDoc® and AsciiDoc Language™ are trademarks of the Eclipse Foundation, Inc.</p>
   </div>
   <div class="footer-thanks">