docs(ngCookies): use GFM code-snippet rather than example tag

The example tag creates a big ugly white rectangle on the docs page, and this is not very helpful
and kind of looks bad. So GFM snippets are a better way to go.

This fix also removes the unnecessary example heading from the $cookieStore page, as there has not
been an example use of $cookieStore for 2 years now.

Closes #7279