Add `^category` front matter to all language, types, and reference doc pages for site navigation restructuring. Replace `:::wren` and `:::java` code block annotations with `:::dart` across multiple files. Create new stub pages for core-libraries, embedding-api, and contributing under the reference category. Remove the old embedding and getting-involved stub pages.
Replace the placeholder TODO stub with full documentation covering function creation, block arguments, parameters, closures, and usage patterns. The new content includes examples of block argument syntax, method definitions accepting functions, and passing existing function objects as arguments.
