docs: render sidebar nav on landing page

The default pydata-sphinx-theme sidebar-nav-bs starts at the current
top-level section, so the root index — which has no parent section —
ends up with an empty sidebar. The theme's layout also explicitly
filters sidebar-nav-bs out of the sidebar list when suppress_sidebar_
toctree() returns true (which it does for root pages), so simply
overriding sidebar-nav-bs.html in templates doesn't help.

Add a sidebar-globaltoc.html template that calls Sphinx's toctree()
global directly to render the full document tree, and wire it through
html_sidebars under a name the theme's suppress filter doesn't strip.
Landing page now shows User Guide / Contributor Guide / API Reference
in the sidebar with the current section expanded on inner pages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/_templates/sidebar-globaltoc.html

@@ -0,0 +1,15 @@
+{# Overrides pydata-sphinx-theme's sidebar-nav-bs to render the global
+   document toctree on every page (including the landing page). The
+   stock template's generate_toctree_html starts at the current
+   section, which leaves the root index with no sidebar at all. Using
+   the toctree() Jinja global instead always shows the top-level
+   structure, with the current section expanded and highlighted. #}
+<nav class="bd-docs-nav bd-links" aria-label="{{ _('Section Navigation') }}">
+  <p class="bd-links__title" role="heading" aria-level="1">{{ _("Section Navigation") }}</p>
+  <div class="bd-toc-item navbar-nav">
+    {{ toctree(maxdepth=theme_navigation_depth | int,
+                collapse=theme_collapse_navigation | tobool,
+                includehidden=True,
+                titles_only=True) }}
+  </div>
+</nav>

docs/source/conf.py

@@ -168,6 +168,10 @@ def setup(sphinx) -> None:
 
 html_css_files = ["theme_overrides.css"]
 
+html_sidebars = {
+    "**": ["sidebar-globaltoc"],
+}
+
 # tell myst_parser to auto-generate anchor links for headers h1, h2, h3
 myst_heading_anchors = 3