Added theme_switcher (pandas-dev#48703)

* Added theme_switcher in navbar_end

Added theme_switcher in  navbar_end to display the Dark/Light theme switcher

* added space after comma

Author: deepaksirohiwal

doc/source/conf.py

@@ -245,7 +245,7 @@
     "twitter_url": "https://twitter.com/pandas_dev",
     "google_analytics_id": "UA-27880019-2",
     "logo": {"image_dark": "https://pandas.pydata.org/static/img/pandas_white.svg"},
-    "navbar_end": ["version-switcher", "navbar-icon-links"],
+    "navbar_end": ["version-switcher", "theme-switcher", "navbar-icon-links"],
     "switcher": {
         "json_url": "/versions.json",
         "version_match": switcher_version,