Nix Flake Prefetch Examples and Description

1st chunk of `src/nix/flake-prefetch.md`

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# Examples

* Download a tarball and unpack it:

  ```console
  # nix flake prefetch https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz --out-link ./result
  Downloaded 'https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz?narHash=sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY='
  to '/nix/store/sl5vvk8mb4ma1sjyy03kwpvkz50hd22d-source' (hash
  'sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY=').

  # cat ./result/README
  Linux kernel
  …
  ```

* Download the `dwarffs` flake (looked up in the flake registry):

  ```console
  # nix flake prefetch dwarffs --json
  {"hash":"sha256-VHg3MYVgQ12LeRSU2PSoDeKlSPD8PYYEFxxwkVVDRd0="
  ,"storePath":"/nix/store/hang3792qwdmm2n0d9nsrs5n6bsws6kv-source"}
  ```

# Description

This command downloads the source tree denoted by flake reference
*flake-url*. Note that this does not need to be a flake (i.e. it does
not have to contain a `flake.nix` file).

)""

Title: Nix Flake Prefetch Examples and Description

Summary

This section provides examples of using the `nix flake prefetch` command to download source trees, including downloading a tarball, unpacking it, and downloading a flake from the flake registry. It also describes the functionality of the command, noting that the downloaded source tree does not necessarily need to be a flake.