US: Best practices for NixOS modules implementation

[imincik]

Description

As a NGIpkgs contributor, I want to follow documented best practices when implementing NixOS modules so that all NGI modules are correct, consistent and secure.

Acceptance criteria

• Given < some context >, when < some action is carried out >, then < set of observable outcomes should occur >.

Notes

Document best practices for:

• Using secrets in systemd services
• Enabling DB or other third-party services for a project
• PostgreSQL configuration for local and remote host
• Django application
• Ruby application
• ...

[phanirithvij]

If there were such a thing for nixpkgs, we could amend it for ngipkgs but I believe there are no such guidelines.

Related but NixOS/nixpkgs#387072 doesn't discuss anything about technical best practices.

Also additional context: this arised from the recent discussion in pdfing module #1813 (comment) And also #1791 (review)

[imincik]

Notion document : https://www.notion.so/nixos-foundation/Nix-NGI-best-practices-for-NixOS-modules-implementation-2c559d49e1be80a8a499f21abb203d6f

[imincik]

PR based on https://www.notion.so/nixos-foundation/17a59d49e1be808a9695fcf86d5b8766?v=596fafe162844f768c68ad3b7c7baa92&p=2c559d49e1be80a8a499f21abb203d6f&pm=s :