[doc] adds alabaster source

docs/_templates/alabaster/relations.html

@@ -0,0 +1,21 @@
+<div class="relations">
+<h3>Related Topics</h3>
+<ul>
+  <li><a href="{{ pathto(master_doc) }}">Documentation overview</a><ul>
+  {%- for parent in parents %}
+  <li><a href="{{ parent.link|e }}">{{ parent.title }}</a><ul>
+  {%- endfor %}
+    {%- if prev %}
+      <li>Previous: <a href="{{ prev.link|e }}" title="{{ _('previous chapter')
+        }}">{{ prev.title }}</a></li>
+    {%- endif %}
+    {%- if next %}
+      <li>Next: <a href="{{ next.link|e }}" title="{{ _('next chapter')
+        }}">{{ next.title }}</a></li>
+    {%- endif %}
+  {%- for parent in parents %}
+  </ul></li>
+  {%- endfor %}
+  </ul></li>
+</ul>
+</div>