Add assignment policy to "Preparing a Pull Request" documentation

[adi-ray]

p5.js version

No response

What is your operating system?

Windows

Web browser and version

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Actual Behavior

• The "first assigned, first served" policy is mentioned only in pinned issues and not in the "Preparing a Pull Request" doc.
• New contributors may not see this policy before submitting PRs.

Expected Behavior

• Add a clear bullet point to the "Before Submitting a Pull Request" section in contributor_docs/preparing_a_pull_request.md that explains the assignment policy.
• For exact content, take reference from issue 💡 Notes to Keep in Mind 💡 #3484 .

Steps to reproduce

This issue was identified when PR #3538 was opened for an already-assigned issue.
Looking through our contributor docs, I'm realizing that this guideline is noted only within the pinned issue and not in the "Preparing a Pull Request" doc. I'm sorry that we couldn't get this in and for this discrepancy, but if you'd like, please feel free to open an issue and pull request to amend this section if you'd like to give a different issue a try!

Originally posted by @raclim in #3538 (comment)

[raclim]

Thanks so much for going ahead with opening this @adi-ray! Please feel free to go ahead with working on this issue if you'd like!

[adi-ray]

Thanks so much for going ahead with opening this @adi-ray! Please feel free to go ahead with working on this issue if you'd like!

Thank you @raclim for the quick approval! I'll work on this issue and will raise a PR soon.