chore: add preview and testing and notes section to pr template

Author: zimeg

.github/pull_request_template.md

@@ -6,6 +6,18 @@
 
 (Please describe the goal of this pull request and mention any related issue numbers)
 
+### Preview
+
+(Add screenshots, GIFs, or recordings that show the changes)
+
+### Testing
+
+(List the steps used to verify these changes)
+
+### Notes
+
+(Add any additional context, trade-offs, or follow-up items)
+
 ### Requirements
 
 - [ ] I've read and understood the [Contributing Guidelines](https://github.com/slackapi/slack-cli/blob/main/.github/CONTRIBUTING.md) and have done my best effort to follow them.