Add design guidelines on site-wide banner

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

Locate the site's style guide and review the existing common site-wide banner examples, including the PyCharm fundraiser, Django survey, fundraising, and Fellowship banners. Document the reviewed design, recommended banner length, and CTA-button usage, with the recurring banners as examples; done means the style guide contains actionable guidance.

Written by the indexing model from the issue text.