Add Github CHERRY_PICK pull request template

Author: Kyle-Ye

.github/PULL_REQUEST_TEMPLATE/CHERRY_PICK.md

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+- **Explanation**: _A description of the issue being fixed or enhancement being made. This can be brief, but it should be clear._
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+- **Scope**: _An assessment of the impact/importance of the change. For example, is the change a source-breaking language change, etc._
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+- **GitHub Issue**: _The issue number if the change fixes/implements an issue/enhancement on [GitHub Issues](https://github.com/apple/swift-docc/issues)._
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+- **Risk**: _What is the (specific) risk to the release for taking this change?_
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+- **Testing**: _What specific testing has been done or needs to be done to further validate any impact of this change?_
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+- **Reviewer**: _One or more code owners for the impacted components should review the change. Technical review can be delegated by a code owner or otherwise requested as deemed appropriate or useful._