Add a GitHub logo to the Doxygen footer

This logo is with a link to the Upa URL repository.

Doxyfile

@@ -1382,7 +1382,7 @@ HTML_HEADER            = doc/header.html
 # that doxygen normally uses.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_FOOTER            =
+HTML_FOOTER            = doc/footer.html
 
 # The HTML_STYLESHEET tag can be used to specify a user-defined cascading style
 # sheet that is used by each HTML page. It can be used to fine-tune the look of

doc/common/version-select.css

@@ -10,3 +10,18 @@
   cursor: pointer;
   line-height: inherit;
 }
+
+.github-mark {
+  display: inline-block;
+  width: 20px;
+  height: 20px;
+  background-color: var(--page-background-color);
+  background-image: url(github-mark.svg);
+  background-repeat: no-repeat;
+  background-size: 20px;
+  vertical-align: middle;
+}
+
+html.dark-mode .github-mark {
+  background-image: url(github-mark-white.svg);
+}

doc/footer.html

@@ -5,6 +5,7 @@
   <ul>
     $navpath
     <li class="footer">$generatedby <a href="https://www.doxygen.org/index.html"><img class="footer" src="$relpath^doxygen.svg" width="104" height="31" alt="doxygen"/></a> $doxygenversion </li>
+    <li class="footer"><a target="_blank" href="https://github.com/upa-url/upa"><span class="github-mark"></span></a></li>
   </ul>
 </div>
 <!--END GENERATE_TREEVIEW-->