theme: Use full title in related content sidebar

If defined, params.altTitle will be used instead. This gives us some
flexibility.

Closes #2965

layouts/partials/layouts/related.html

@@ -14,7 +14,7 @@
         <a
           class="text-sm text-blue-600 hover:text-blue-500 dark:text-blue-500 dark:hover:text-blue-400"
           href="{{ .RelPermalink }}"
-          >{{ .LinkTitle }}</a
+          >{{ or .Params.altTitle .Title }}</a
         >
       </li>
     {{- end }}