Using Nixpkgs

1st chunk of `doc/using-nixpkgs.md`

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# Using Nixpkgs {#part-using}

```{=include=} chapters
using/platform-support.chapter.md
using/configuration.chapter.md
using/overlays.chapter.md
using/overrides.chapter.md
```

Title: Using Nixpkgs

Summary

This section provides information on how to use Nixpkgs, covering topics such as platform support, configuration, overlays, and overrides.