Improve the documentation for the `guest_dispatch_function` in the C and Rust Guest APIs

The hyperlight guest library defines a function called `guest_dispatch_function` which allows the guest to dynamically dispatch calls to functions, this function is not really documented anywhere. We should document this function more clearly explaining its purpose and how to use it.

More generally we should improve [this](https://github.com/hyperlight-dev/hyperlight/blob/main/docs/how-to-build-a-hyperlight-guest-binary.md) documentation to describe how to build a guest.