Add copilot instructions for documentation checks

[dsarmany]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.95%. Comparing base (03c7e0f) to head (3e6a148).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #189      +/-   ##
===========================================
- Coverage    62.33%   61.95%   -0.39%     
===========================================
  Files          303      303              
  Lines        11601    11673      +72     
  Branches      1036     1046      +10     
===========================================
  Hits          7232     7232              
- Misses        4369     4441      +72     

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[Copilot]

Pull request overview

Adds repository-level Copilot instructions to enforce documentation synchronization checks scoped to the mars2grib implementation and its tests.

Changes:

• Introduces .github/copilot-instructions.md with a rule to check documentation sync for changes under selected directories.
• Defines what “documentation in sync” means and prescribes PR review reporting behavior for those paths.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.