[Docs] Regenerate setup prompts and clarify SDK installation steps

- Updated the setup documentation for Stack Auth, ensuring consistency across guides and snippets.
- Enhanced instructions for installing SDKs, emphasizing the correct packages for various frameworks, including Next.js, React, and TanStack Start.
- Clarified the process for initializing Stack App objects and setting up environment variables, particularly for frontend and backend configurations.
- Regenerated `setup.mdx` and `home-prompt-island.jsx` to reflect the latest changes in the setup prompts.

Author: mantrakp04