Fix invalid URLs in version-switcher using dirhtml builder (#2004)

Fix #2002.

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Co-authored-by: M Bussonnier <[email protected]>

Author: cgohlke

src/pydata_sphinx_theme/assets/scripts/pydata-sphinx-theme.js

@@ -346,8 +346,8 @@ var setupSearchButtons = () => {
  */
 var getCurrentUrlPath = () => {
   if (DOCUMENTATION_OPTIONS.BUILDER == "dirhtml") {
-    return DOCUMENTATION_OPTIONS.pagename == "index"
-      ? `/`
+    return DOCUMENTATION_OPTIONS.pagename.endsWith("index")
+      ? `${DOCUMENTATION_OPTIONS.pagename.substring(0, DOCUMENTATION_OPTIONS.pagename.length - 'index'.length)}`
       : `${DOCUMENTATION_OPTIONS.pagename}/`;
   }
   return `${DOCUMENTATION_OPTIONS.pagename}.html`;