[docs] Improve sidebar (#12461)

- Remove "Site navigation" header
  - this is already contextually clear (especially since separators were added in #12439)
- Hide "On this page" if `display_toc is False`
- Format "On this page" header similar to top-level site-nav sections

doc/_themes/sphinx13/layout.html

@@ -34,12 +34,13 @@
 <div class="document">
   <div class="sphinxsidebar" role="navigation" aria-label="Main">
     {%- include "searchfield.html" %}
-    <div class="sphinxsidebar-navigation__contents">
-      <h3>{{ _('On this page') }}</h3>
-      {{ toc }}
-    </div>
+    {%- if display_toc %}
+      <div class="sphinxsidebar-navigation__contents">
+        <h3>{{ _('On this page') }}</h3>
+        {{ toc }}
+      </div>
+    {%- endif %}
     <div class="sphinxsidebar-navigation__pages">
-      <h3>{{ _('Site navigation') }}</h3>
       {{ toctree(includehidden=True, maxdepth=3, titles_only=True) }}
     </div>
   </div>

doc/_themes/sphinx13/static/sphinx13.css

@@ -110,7 +110,8 @@ div.sphinxsidebar input {
 }
 
 div.sphinxsidebar h3 {
-    font-size: 1.5em;
+    font-size: 1.2em;
+    font-weight: 300;
     margin-top: 0;
     margin-bottom: 0.5em;
     padding-top: 0.5em;