Create Block: Add documentation for custom block namespace

[Infinite-Null]

What?

Adds a brief section about block namespaces to the Create Block package documentation. Closes: #42854

Why?

When using @wordpress/create-block, the tool generates blocks with the default create-block namespace (e.g., create-block/my-block). This causes issues because:

• Multiple plugins end up using the same namespace
• Developers often don't realize they need to change it until after plugin submission

How?

Added a concise "Block Namespace" section under Usage that:

• Explains that create-block is the default namespace
• Shows how to specify a custom namespace using the --namespace option
• Indicates where to update the namespace in existing blocks

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Co-authored-by: mburridge <[email protected]>
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