add favicon to documentation

This change copies the Wagtail admin favicon.ico (from
wagtail/admin/static_src/wagtailadmin/images) into the Sphinx build
directory so that built docs use the same favicon as the admin.

Makes use of [the Sphinx `html_favicon` setting](http://www.sphinx-doc.org/en/1.5.1/config.html#confval-html_favicon)
which adds this line to the generated HTML:

```html
<link rel="shortcut icon" href="_static/favicon.ico"/>
```

Fixes #4340.

docs/conf.py

@@ -151,7 +151,7 @@ html_logo = 'logo.png'
 # The name of an image file (within the static path) to use as favicon of the
 # docs.  This file should be a Windows icon file (.ico) being 16x16 or 32x32
 # pixels large.
-#html_favicon = None
+html_favicon = 'favicon.ico'
 
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,