docs(nix): add installation instructions for Nix

[0x61nas]

Description

Added the installation instructions for Nix/NixOS to the website

Motivation and Context

It would be nice to add instructions for Nix

• Closes Add installation instructions for Nix #668

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[welcome]

Thanks for opening this pull request! Please check out our contributing guidelines! ⛰️

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 36.89%. Comparing base (8ffc054) to head (6f09184).
⚠️ Report is 425 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #669   +/-   ##
=======================================
  Coverage   36.89%   36.89%           
=======================================
  Files          18       18           
  Lines        1350     1350           
=======================================
  Hits          498      498           
  Misses        852      852           

Flag	Coverage Δ
unit-tests	36.89% <ø> (ø)

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[orhun]

Thanks!

[welcome]

Congrats on merging your first pull request! ⛰️