User guide on time in MUSE

[tsmbland]

Description

Can probably wait until after v2.2.0

Fixes # (issue)

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 90.88%. Comparing base (7cd741d) to head (3e37662).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1515      +/-   ##
==========================================
- Coverage   90.95%   90.88%   -0.07%     
==========================================
  Files          61       61              
  Lines        9050     9050              
  Branches     9050     9050              
==========================================
- Hits         8231     8225       -6     
- Misses        500      506       +6     
  Partials      319      319              

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[Copilot]

Pull request overview

Adds a dedicated “Time” section to the Model Description documentation, explaining how MUSE2 represents time across the long-term horizon (milestone years) and within-year operational resolution (seasons/time slices). This improves discoverability and provides a single reference point for time-slice concepts used throughout model configuration and constraints.

Changes:

• Added new documentation page docs/model/time.md describing milestone years, seasons/time slices, and time-slice selections.
• Linked the new “Time” page into the documentation navigation (docs/SUMMARY.md) and referenced it from the model overview (docs/model/README.md).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/SUMMARY.md	Adds “Time” to the Model Description section of the docs navigation.
docs/model/time.md	New detailed documentation page describing MUSE2’s time representation and configuration.
docs/model/README.md	Adds a pointer directing readers to the new “Time” page for more detail.

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