nix registry list: Displaying the Contents of All Registries

1st chunk of `src/nix/registry-list.md`

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# Examples

* Show the contents of all registries:

  ```console
  # nix registry list
  user   flake:dwarffs github:edolstra/dwarffs/d181d714fd36eb06f4992a1997cd5601e26db8f5
  system flake:nixpkgs path:/nix/store/fxl9mrm5xvzam0lxi9ygdmksskx4qq8s-source?lastModified=1605220118&narHash=sha256-Und10ixH1WuW0XHYMxxuHRohKYb45R%2fT8CwZuLd2D2Q=&rev=3090c65041104931adda7625d37fa874b2b5c124
  global flake:blender-bin github:edolstra/nix-warez?dir=blender
  global flake:dwarffs github:edolstra/dwarffs
  …
  ```

# Description

This command displays the contents of all registries on standard
output. Each line represents one registry entry in the format *type*
*from* *to*, where *type* denotes the registry containing the entry:

* `flags`: entries specified on the command line using `--override-flake`.
* `user`: the user registry.
* `system`: the system registry.
* `global`: the global registry.

See the [`nix registry` manual page](./nix3-registry.md) for more details.

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Title: nix registry list: Displaying the Contents of All Registries

Summary

The `nix registry list` command displays the contents of all Nix registries (user, system, and global) to standard output. Each line represents a registry entry in the format 'type from to', where 'type' indicates the registry containing the entry (flags, user, system, or global). The 'from' and 'to' fields specify the mapping defined by the registry entry. This command provides a way to inspect the configured flake registries and their mappings.