Enhancement: Add PR Templates

[Grochocinski]

Linked Issue (Required)

Closes #174

Summary

Introduces a set of PR templates to standardize the contribution workflow

Proposed Changes

• Add template menu (.github/pull_request_template) to act as a landing page, allowing users to select the proper template for their PR
• Add solver (new_solver.md) and model/problem (new_model_problem.md) templates to give a baseline overview of new solvers/models/problems
• Add a bug fix template (bug_fix.md) to mandate bug fix explanations and standardize testing
• Add an enhancement template (enhancement.md) to provide a structure for general improvements and refactoring

Impact

• Performance improvement
• Code cleanup / Refactoring
• Documentation update

Breaking Changes

• No
• Yes (Explain: )

Checklist

• ruff check passes on my code
• ty check passes on my code
• My code follows the project's style guidelines
• I have updated documentation as needed
• I have added tests to cover the new logic