Improve PR template

[Genyus]

Description

• Adds additional change type options
• Tweaks CONTRIBUTING guide

Motivation and Context

This ensures users can indicate all appropriate change types included in their PR

How has this been tested?

Can't be tested until after merging this PR

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Test addition/modification (change that adds or modifies tests)
• Exercise solution (implementation of an exercise solution)
• Documentation (change to documentation)
• Refactoring (improvement to code structure without changing functionality)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.