docs: enhance Definition of Done

[M-Adam-Hus]

Updating the pull request template to align with a new 'Definition of Done'.

Key changes include:

• Added a mandatory 'Purpose' section to explain the 'why'.
• Added specific instructions for testing steps.
• Updated the checklist to be more engaging and focused on code quality and self-review.

Description of change

Please write a summary of your changes and why you made them.

Links to any relevant issues

Be sure to reference any related issues by adding fixes issue #.

How the change has been tested

Describe the tests that you ran to verify your changes.
Make sure to provide instructions for the maintainer as well as any relevant configurations.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have successfully tested this change in a docker environment