Add Nix flake for ergonomic development and reproducible builds

[RLangendam]

Objective

• Simplify setting up a development environment
• Add support for reproducible builds

Solution

• Add a nix flake and put the flake.nix under version control
• Update the documentation to reflex the proper use of nix flakes for development

Testing

• cargo build --all
• cargo test --all (only the already failing tests on main continue failing)

[mockersf]

Opposed to this