PYTHON-5576: add PR template to mongo-python-driver

[sleepyStick]

Summary

Adds a pull request template to this repository based on our new dbx-python code review policy.

Changes in this PR

• pull_request_template.md

Test Plan

merged this branch into my fork's main and made a test PR to confirm.
if you'd like to see it in action yourself, click here, and then click "create pull request" (no need to actually create it after that though)

Screenshots (optional)

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
  not necessary
• Is the intention of the code captured in relevant tests?
  not necessary
• If there are new TODOs, has a related JIRA ticket been created?
  not necessary

Checklist for Reviewer { @Jibola , @caseyclements }

• 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?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer

Any typos? Formatting changes?

Note: I'm not sure if the capitalization actually made a difference but github documentation says the template name should be in all lowercase.