Add documentation for commodity price calculations

[tsmbland]

Description

Please include a summary of the change and which issue is fixed (if any). Please also
include relevant motivation and context. List any dependencies that are required for
this change.

Fixes #1034

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.75%. Comparing base (651b4e2) to head (01473c0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1362   +/-   ##
=======================================
  Coverage   89.75%   89.75%           
=======================================
  Files          58       58           
  Lines        8529     8529           
  Branches     8529     8529           
=======================================
  Hits         7655     7655           
  Misses        561      561           
  Partials      313      313           

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