doc: enable three-way dark-mode switch

dark-mode-toggle now allows to also set current mode to "system", which
allows people to use the system-wide dark mode setting rather than
having to definitively choose between light and dark mode once they've
used the button once.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/_templates/breadcrumbs.html

@@ -15,7 +15,7 @@
 {% endblock %}
 {%- block breadcrumbs_aside %}
   <li class="wy-breadcrumbs-aside">
-    <dark-mode-toggle id="dark-mode-toggle" appearance="toggle" permanent="true"/>
+    <dark-mode-toggle id="dark-mode-toggle" appearance="three-way" permanent="true"/>
   </li>
   <li class="wy-breadcrumbs-aside">
     {%- if display_gh_links %}