Change the logo and add a dark option (#716)

Author: angeloskath

docs/src/_static/mlx_logo.png

@@ -49,10 +49,12 @@
     "repository_url": "https://github.com/ml-explore/mlx",
     "use_repository_button": True,
     "navigation_with_keys": False,
+    "logo": {
+        "image_light": "_static/mlx_logo.png",
+        "image_dark": "_static/mlx_logo_dark.png",
+    },
 }
 
-html_logo = "_static/mlx_logo.png"
-
 
 # -- Options for HTMLHelp output ---------------------------------------------