Add Hugo shortcodes and CSS to Hugo theme to allow notice blocks to be
added to the markdown. There are 4 types available:

- warning
- info
- note
- tip

By adding the following hugo shortcode around some text a formatted box is
added to the HTML output:

    {{% notice info %}}
    Text for notice
    {{% /notice %}}

This will be useful for highlighting important information.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>