doc: update footer to display correctly

Update the footer to match what has been changed in the Furo theme:
pradyunsg/furo@8eba649

Signed-off-by: Ruth Fuchss <ruth.fuchss@canonical.com>
(cherry picked from commit 2fdd931)
Signed-off-by: Simon Deziel <simon.deziel@canonical.com>

doc/.sphinx/_templates/footer.html

@@ -9,12 +9,12 @@
         </div>
         <div class="title">{{ next.title }}</div>
       </div>
-      <svg><use href="#svg-arrow-right"></use></svg>
+      <svg class="furo-related-icon"><use href="#svg-arrow-right"></use></svg>
     </a>
   {%- endif %}
   {% if prev -%}
     <a class="prev-page" href="{{ prev.link }}">
-      <svg><use href="#svg-arrow-right"></use></svg>
+      <svg class="furo-related-icon"><use href="#svg-arrow-right"></use></svg>
       <div class="page-info">
         <div class="context">
           <span>{{ _("Previous") }}</span>