Display the RTD version switcher on the index page

This uses a custom template for the index page sidebar to ensure that
the sidebar remains visible, so that the RTD version switcher is
displayed.

doc/conf.py

@@ -201,6 +201,13 @@ html_logo = 'logo.png'
 
 # Custom sidebar templates, maps document names to template names.
 # html_sidebars = {}
+html_sidebars = {
+    'index': [
+        'sidebar-pypfilt.html',
+        'sidebar-nav-bs.html',
+        'sidebar-ethical-ads.html',
+    ],
+}
 
 # Additional templates that should be rendered to pages, maps page names to
 # template names.

doc/templates/sidebar-pypfilt.html

+<!--
+  Insert an empty div to ensure the sidebar remains visible on the index page,
+  to ensure that the Read The Docs version switcher is displayed.
+
+  See these related issues:
+
+  - https://github.com/pydata/pydata-sphinx-theme/issues/221
+  - https://github.com/pydata/pydata-sphinx-theme/issues/705
+  - https://github.com/pydata/pydata-sphinx-theme/issues/1092
+-->
+<div></div>