Add Nix and Conda installation details

[colindean]

Progress toward #805. I omitted Scoop because its PR hasn't been merged yet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.57%. Comparing base (ff3a674) to head (0ba438b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #874      +/-   ##
==========================================
+ Coverage   89.47%   89.57%   +0.09%     
==========================================
  Files          60       60              
  Lines       11215    11320     +105     
==========================================
+ Hits        10035    10140     +105     
  Misses       1180     1180              

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[j178]

Thank you!