Update post-release steps in CONTRIBUTING.md

[jeffpaul]

What?

This pull request updates the release process documentation in CONTRIBUTING.md to clarify announcement steps and contributor recognition. The most important changes are:

Release announcement process:

• The release announcement step now specifies publishing on Make/AI, cross-posting to Make/Core and Make/Test, and provides an example link for reference.

Contributor recognition:

• Added a step to grant new contributors the Core AI Contributor profile badge after a release.

Why?

Adds a bit more clarity and steps to our release process to continually improve our docs (mainly because I don't want to have to be responsible for all releases).

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

Test using WordPress Playground

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[github-actions]

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Co-authored-by: jeffpaul <[email protected]>
Co-authored-by: dkotter <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.