Update docs vars to work with RTD changes (#6410)

docs/conf.py

@@ -139,6 +139,9 @@ html_context = {
     "github_user": "Cog-Creators",
     "github_repo": "Red-DiscordBot",
     "github_version": "V3/develop",
+    "version_slug": os.environ.get("READTHEDOCS_VERSION", ""),
+    "rtd_language": os.environ.get("READTHEDOCS_LANGUAGE", ""),
+    "READTHEDOCS": os.environ.get("READTHEDOCS", "") == "True",
 }
 
 # Add any paths that contain custom static files (such as style sheets) here,