[FEATURE]: Add documentation for self-hosted deployment (Nginx and Cloudflare Tunnel)

### Feature and its Use Cases

Documentation covering how to expose a self-hosted ThruBox-Server instance to the internet after running the Docker container locally.

How would users benefit from it: Users get clear, ready-made guidance for the last step of self-hosting, instead of having to research reverse proxy or tunnel setup on their own.

What scenarios would this feature address: A user runs the Docker container locally (e.g., port 3000 exposed) and wants to make it accessible from the internet using Nginx or Cloudflare Tunnel.

### Additional Context

Scope clarification: we are responsible for providing the Docker container/code that runs locally. Exposing it to the internet is left to the user, guided by our documentation. A fully automatic built-in exposure mechanism is not feasible, since it would require admin access to the user's router, DNS, etc.

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates