Assests that can be used across R Contributor projects

• css/styles.css: This file describes the presentation of a document in HTML.
• img/Rlogo-hex.png: Hex style logo for R.
• _brand.yml: From the brand.yml website, "brand.yml is a simple, portable YAML file that codifies your company’s brand guidelines into a format that can be used by Quarto, Python and R tooling to create branded outputs."