docs(contribution-guidelines): add guidance on a common "error" (#32983)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

Anecdotally, contributors often encounter the "This branch is out-of-date with the base branch" message, which can be confusing. Since I couldn’t find a clear explanation, I sought clarification from one of the admins in [this comment](#32889 (comment)). I’ve summarized their guidance to help other contributors navigate this issue more easily.




### Description of changes

Added clarification on a common "error" in the contributor guidelines.




### Describe any new or updated permissions being added




### Description of how you validated changes

An admin provided guidance on the issue, and it resolved the problem effectively in my case.




### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -718,6 +718,10 @@ To make this easier we have a `pr/needs-review` label that we can add to each
 PR. If you do not see this label on your PR then it means that something needs
 to be fixed before it can be reviewed.
 
+> [!NOTE]  
+> The `aws-cdk` repository is frequently updated, so PR branches may quickly become out-of-date, showing "This branch is out-of-date with the base branch." This is not an issue as long as there are no conflicts with the newly merged commits. Once the PR is approved, our automation will update it with the latest `main` branch and handle the merge. No action is needed on your part.
+
+
 #### Adding construct runtime dependencies
 
 Any tool that is not part of the CDK, and needs to be used by a construct during