Add docs for single-metallicity calculations

[aphearin]

Add docs for single-metallicity calculations

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (21bdda2) 59.40% compared to head (c07950f) 59.40%. Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #79 +/- ## ======================================= Coverage 59.40% 59.40% ======================================= Files 93 93 Lines 6565 6565 ======================================= Hits 3900 3900 Misses 2665 2665 ```

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[aphearin]

@matroxel