docs: address Copilot review feedback on sidebar config

Two small follow-ups from the Copilot reviewer on #1578:

- Append .html to the html_sidebars entry. Sphinx's Jinja loader
  resolves both "sidebar-globaltoc" and "sidebar-globaltoc.html" to
  the same template, but the explicit form is closer to the spelling
  in the Sphinx docs and is harder to misread.
- Update the inline comment in sidebar-globaltoc.html that still
  claimed show_nav_level=1 after we bumped it to 2 in conf.py. Now
  describes the variable wiring instead of hard-coding a number that
  has to be kept in sync with conf.py.

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

Author: timsaucer

docs/source/_templates/sidebar-globaltoc.html

@@ -13,8 +13,9 @@
   <div class="bd-toc-item navbar-nav">
     {# collapse=False so nested <ul>s render into the DOM for every
        branch; the theme's JS then adds the expand/collapse chevrons
-       on top. show_nav_level=1 keeps deeper levels collapsed by
-       default so the landing-page sidebar isn't a wall of links. #}
+       on top. show_nav_level is driven by theme_show_nav_level (set
+       in conf.py) so callers can adjust how deep the sidebar opens
+       by default without editing this template. #}
     {{- generate_toctree_html(
       "sidebar",
       startdepth=0,

docs/source/conf.py

@@ -165,7 +165,7 @@ def setup(sphinx) -> None:
 html_css_files = ["theme_overrides.css"]
 
 html_sidebars = {
-    "**": ["sidebar-globaltoc"],
+    "**": ["sidebar-globaltoc.html"],
 }
 
 # tell myst_parser to auto-generate anchor links for headers h1, h2, h3