[Queues] add r2 event notifications disclaimer in src/content/docs/queues/get-started.mdx

[LA]

Summary

This PR adds a disclaimer to the queues get started documentation. The documentation currently states that:

You must create at least one producer Worker to publish messages onto your queue.

However, if you are using R2 Bucket Event Notifications, then you do not need a producer Worker. It does not state this in either the Queues or the Event Notifications documentation as far as I can tell.

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[KianNH]

Thanks for adding this!

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