docs: run haskell application as a process

We already do this in [nammayatri](https://github.com/nammayatri/nammayatri/blob/main/Backend/nix/services/nammayatri.nix)

There are two variants, `nix run .#run-mobility-stack-nix` and `nix run .#run-mobility-stack-dev`:

https://github.com/nammayatri/nammayatri/blob/57be9e73731a42a8ddb6b6915ceb048283c6e0fe/Backend/nix/run-mobility-stack.nix#L16-L29

the dev variant runs `cabal build` followed by `cabal run`, while the nix variant uses the `apps` exported by flake to start the haskell application.

This can be part of a tutorial series