fix(Other): Add `delete me warning` in PR template

[sihyung-maxim]

Description

This PR updates the PR template so users know they need to delete the unnecessary parts.

Checklist Before Requesting Review

• [ ] PR Title follows correct guidelines.
• [ ] Description of changes and all other relevant information.
• [ ] (Optional) Link any related GitHub issues using a keyword
• [ ] (Optional) Provide info on any relevant functional testing/validation. For API changes or significant features, this is not optional.