Supported Platforms for Nix

1st chunk of `doc/manual/source/installation/supported-platforms.md`

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# Supported Platforms

Nix is currently supported on the following platforms:

  - Linux (i686, x86\_64, aarch64).

  - macOS (x86\_64, aarch64).

Title: Supported Platforms for Nix

Summary

Nix is supported on Linux (i686, x86_64, aarch64) and macOS (x86_64, aarch64).