Add pull request template with testing checklist

[karthikvetrivel]

Description

Add a PR template to ensure contributors document how changes were tested and for general documentation purposes.

Context (optional)

We currently have no PR template. This makes it easy for PRs to slip through without adequate testing documentation.

Testing

• [ ] Additional unit tests (make test)
• [ ] E2E tests (describe below)
• [ ] Manual cluster testing (describe below)
• [x] N/A (docs, CI config, etc.)

Test details: Adding GitHub config file only - no code changes.