[docs] raise z-index block (#10843)

Co-authored-by: Richard Liaw <rliaw@berkeley.edu>
Co-authored-by: Sumanth Ratna <sumanthratna@gmail.com>

doc/source/_static/css/custom.css

@@ -52,3 +52,8 @@ img.inline-figure {
   /* Make version warning clickable */
   z-index: 1;
 }
+
+/* Adjustment to Version block */
+.rst-versions {
+  z-index: 1200;
+}