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.
