Backport PR pandas-dev#40761: DOC: collapse subpages in sidebar for API reference docs (pandas-dev#40922)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: meeseeksmachine

doc/_templates/sidebar-nav-bs.html

@@ -0,0 +1,9 @@
+<nav class="bd-links" id="bd-docs-nav" aria-label="Main navigation">
+  <div class="bd-toc-item active">
+    {% if pagename.startswith("reference") %}
+    {{ generate_nav_html("sidebar", maxdepth=4, collapse=True, includehidden=True, titles_only=True) }}
+    {% else %}
+    {{ generate_nav_html("sidebar", maxdepth=4, collapse=False, includehidden=True, titles_only=True) }}
+    {% endif %}
+  </div>
+</nav>