doc: add section on swift

[stephank]

Description of changes

A first take on documentation for Swift packaging.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[stephank]

Not sure who to ping for this, maybe @trepetti or @NixOS/documentation-team?

[domenkozar]

Having these docs is much better than the current state, so I'm merging.

[fricklerhandwerk]

Reviewed it after the fact, since I was on vacation. @stephank Awesome work, thanks a lot! @figsoda is this template amenable to be added to nix-init?

[figsoda]

nix-init currently doesn't have support for multiple files, so the most it can do atm would be to recognize Package.swift and generate a plain-text-ish template without the dependencies

feel free to create an issue or pull request there

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-44/25546/1