docs: remove outdated Nix flake setup instructions

[sisp]

I've removed the outdated Nix flake setup instructions, as we're not publishing a Nix flake on flakehub.com anymore.

WDYT, @yajo?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.90%. Comparing base (165c85a) to head (102654d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2282      +/-   ##
==========================================
- Coverage   98.00%   97.90%   -0.10%     
==========================================
  Files          55       55              
  Lines        6160     6160              
==========================================
- Hits         6037     6031       -6     
- Misses        123      129       +6     

Flag	Coverage Δ
unittests	97.90% <ø> (-0.10%)	⬇️

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