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.
Add a comprehensive Getting Started guide covering Unix/Mac installation via git clone and make, interactive mode usage with code examples, and script execution with a Mandelbrot fractal demo. Also update the site navigation template to link the new page, add placeholder pages for Embedding, Error Handling, and Getting Involved, fix the index page link from "usage.html" to "embedding.html", and reorder the types section in the nav to list Fibers before Functions.
