Use stable link to Wand docs

There's only one link to Wand's docs, not worth adding an
intersphinx_mapping entry.

docs/advanced_topics/images/animated_gifs.md

@@ -4,7 +4,7 @@ Pillow, Wagtail's default image library, doesn't support animated
 GIFs.
 
 To get animated GIF support, you will have to
-[install Wand](https://docs.wand-py.org/en/0.6.7/guide/install.html).
+[install Wand](https://docs.wand-py.org/en/stable/guide/install.html).
 Wand is a binding to ImageMagick so make sure that has been installed as well.
 
 When installed, Wagtail will automatically use Wand for resizing GIF