Deactivate sidebar for release notes

The release note page is only a structured collection of links to
sub-pages. The sidebar would repeat all the titles of the sub-pages and
thus basically repeat all the content of the page. Even worse, it does
not contain the version sectioning structure of the release notes page
itself. To keep the layout the sidebar gets one empty dummy section.
If we leave the sidebar completely empty, it will be deactivated and
change the layout of the page.

Related to matplotlib#27690 (comment)

Author: timhoffm

doc/_templates/empty_sidebar.html

@@ -0,0 +1 @@
+<div class="sidebar-empty"></div>

doc/conf.py

@@ -536,6 +536,10 @@ def js_tag_with_cache_busting(js):
         "cheatsheet_sidebar.html",
         "donate_sidebar.html",
     ],
+    # no sidebar for release notes, because that page is only a collection of links
+    # to sub-pages. The sidebar would repeat all the titles of the sub-pages and
+    # thus basically repeat all the content of the page.
+    "users/release_notes": ["empty_sidebar.html"],
     # '**': ['localtoc.html', 'pagesource.html']
 }