Add design guidelines on site-wide banner

[sarahboyce]

We often have a requirement to add a site wide banner to advertise something over a limited period of time. Some of these advertisements are recurring. Examples include:

• the PyCharm fundraiser where a discounted PyCharm license is offered with proceeds going to the DSF
• the Django survey
• end of year fundraising push
• request for Fellow applications

It would be good to review the design and add to the style guide, along with guidelines on length, use of the cta button etc. The common banners can be used as examples