Add documentation for commodity price calculations

[tsmbland]

Description

Adds documentation describing the approach for calculating commodity prices. Includes the example laid out in #1128 (comment)

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 (019e9e3).
⚠️ Report is 3 commits behind head on main.

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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[Copilot]

Pull request overview

Adds a new user-facing documentation page describing how MUSE2 calculates commodity prices after dispatch, including an end-to-end worked example for full-cost pricing (per the referenced issues).

Changes:

• Add a new “Commodity Prices” page under the Model Description section.
• Document pricing strategies, cost component breakdowns, ordering logic (including cycles), and a worked example.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
docs/SUMMARY.md	Adds the new “Commodity Prices” page to the mdBook table of contents.
docs/model/prices.md	New documentation page describing pricing strategies and worked examples for commodity price calculation.

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