Create a style guide for Mass.gov/Mayflower design components

As a content author or app developer, would like to have access to a style guide that contextualizes Mayflower and other design elements. I'd like to know what elements are used, where they are applicable, and what components should be utilized for particular functions.

**JIRA ticket #:** DP-8088

No repository files, tests, or entry points are identified in the issue. First clarify where Mayflower and Mass.gov design documentation belongs and which components and use cases are in scope; done means a published style guide explaining applicable elements and recommended components for particular functions.

Written by the indexing model from the issue text.