PYTHON-5401: Refactor PR template for clarity and conciseness

[Jibola]

PYTHON-5401

Changes in this PR

Added pull_request_template.md

Test Plan

Standard process

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[caseyclements]

LGTM!