Switch to new changelog format

[djdameln]

Description

• Updates the changelog to the new format and adds [unreleased] section.
• Add instruction in contributing guidelines.
• Adds a checkbox to the PR template.

Changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which refactors the code base)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the pre-commit style and check guidelines of this project.
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes

[ashwinvaidya17]

Thanks

[samet-akcay]

Thanks. Despite adding some complexity to PR making process, this seems to be a lot more descriptive to users!