[DOCS] Document Nix flake as alternative dev environment setup

[c-vigo]

Description

The project provides a flake.nix with a dev shell containing all required dependencies, plus a .envrc for direnv integration, but neither CONTRIBUTE.md nor README.md mention Nix as a setup option.

Update the Requirements/Setup section in docs/templates/CONTRIBUTE.md.j2 to document:

• Nix flake as an alternative to manual package installation
• The experimental features that must be enabled (nix-command, flakes) — either via ~/.config/nix/nix.conf or the --extra-experimental-features flag
• Optional direnv integration via the existing .envrc

Documentation Type

Update existing documentation

Target Files

• docs/templates/CONTRIBUTE.md.j2 (source template)

Acceptance Criteria

• Template adds a "Nix (alternative)" section alongside the existing Ubuntu/macOS install instructions
• Documents how to enable nix-command and flakes experimental features
• Documents nix develop usage and optional direnv setup
• Generated CONTRIBUTE.md regenerated with just docs

Changelog Category

Changed

Additional Context

The flake.nix and .envrc were added without corresponding documentation. Users encountering Nix for the first time hit error: experimental Nix feature 'nix-command' is disabled with no guidance.