DOC: Fix matching for version switcher

A released version should point to its version, not 'stable', since that
doesn't appear in the version switcher. And devdocs should point to
'dev', since that's what it's called in the JSON.

Author: QuLogic

doc/conf.py

@@ -514,10 +514,9 @@ def js_tag_with_cache_busting(js):
             f"https://matplotlib.org/devdocs/_static/switcher.json?{SHA}"
         ),
         "version_match": (
-            # The start version to show. This must be in switcher.json.
-            # We either go to 'stable' or to 'devdocs'
-            'stable' if matplotlib.__version_info__.releaselevel == 'final'
-            else 'devdocs')
+            matplotlib.__version__
+            if matplotlib.__version_info__.releaselevel == 'final'
+            else 'dev')
     },
     "navbar_end": ["theme-switcher", "version-switcher", "mpl_icon_links"],
     "navbar_persistent": ["search-button"],