Update the pull-request-template.

[rexwangcc]

Purpose

Please link to the issue being fixed by this PR:

• https://app.zenhub.com/workspaces/dcp-backlogs-5ac7bcf9465cb172b77760d9/issues/humancellatlas/secondary-analysis/395

  Changes

  Please list out what major changes were made in this PR to address the issue:

• Update and improve the pull_request_template.md.

  Review Instructions

  Please provide instructions about how should a reviewer test/verify the changes in this PR:

• Preview the markdown file pull_request_template.md. Alternatively, you could look at ME to have an overview!

• Note: This template will be different in Skylab repo since that repo has specific requirements for PRs.

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PR Checklist

Please ensure the following when opening a PR:

• [ ] This PR added or updated tests.
• [x] This PR updated docstrings or documentation.
• [ ] This PR applied (WDL/Python) style guidelines.
• [ ] This PR considered generalizability beyond our own use case.

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Follow-up Discussions

Please append follow-up discussions during the review process below:

• No follow-up discussions.

[rexwangcc]

@samanehsan Updated the template based on your comments! Thanks for the review!