Add serverless shared attributes

[lcawl]

Relates to elastic/docs-content#182

This PR moves the declarations from https://github.com/elastic/docs-content/blob/7602ddbdb3de8fe99b0850423c308f85e2952d6d/serverless/index.asciidoc?plain=1#L13 to the list of shared attributes so that they can be used in all books.

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[colleenmcginnis]

Thanks! This was in my post-migration list along with #3107 and #3108. 🙂 Feel free to delete from the file in /docs-content, too!