Provide a Nix development environment with all dependencies for Core contributors

[arejula27]

It would be very useful to provide a ready-to-use development environment for Bitcoin Core using Nix. This environment should include all the dependencies required to build and run Core, allowing contributors to simply enter the shell and start coding and compiling without additional setup.

This would simplify onboarding for new contributors

A Nix shell (shell.nix) or Nix Flake could achieve this by specifying all build dependencies and environment variables needed for Bitcoin Core development.

[prusnak]

You can use nix develop nixpkgs#X to install the exact build environment of the package X.

No need to add anything to nix-bitcoin.

[arejula27]

Would be interesting to add it on the doc then, I only found one reference to shell and was a local file :v