Update serverless Discover URL

[leemthompo]

👋 We're de-duplicating content in the Elasticsearch serverless docs, and this is an example of a feature that will have its single-source-of-truth in the Kibana docs. I'll need this change merged, in order to get a green build on my PR in the docs-content repo.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @leemthompo? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[natasha-moore-elastic]

LGTM!