chore: update PR checklist

[Manciukic]

Changes

Improve the PR checklist to make it more clear and explicitly ask contributors to run the automated checkstyles.

Reason

The current PR checklist doesn't tell to run the checkstyle explicitly and it's a common reason PRs from external contributors get delayed. I also took the opportunity to refresh it.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkstyle to verify that the PR passes the automated style checks.
• I have described what is done in these changes, why they are needed, and how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs) in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or integration tests.
• I have linked an issue to every new TODO.

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• This functionality cannot be added in rust-vmm.