Add `^category` front matter to branching, classes, error-handling, fibers, functions, lists, looping, maps, method-calls, performance, and qa markdown files. Create new placeholder pages for contributing, core-libraries, and embedding-api under reference category. Remove old embedding and getting-involved pages. Change all `:::wren` code block annotations to `:::dart` across multiple documentation files. Update index title from "Welcome" to "Welcome!" and fix a typo in the Wren class example string. Refine performance page wording for clarity.
Bob Nystrom
818551fd4a