docs: run haskell application as a process

[shivaraj-bh]

We already do this in nammayatri

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