Add docs about the Hosts metrics tab to serverless

[dedemorton]

Ports #3939 to serverless.

Note that there is some content currently missing from the serverless docs. I don't have time to port that content today so I've created an issue here: #3954

Preview is here: https://docs-elastic-odt7mapih-elastic-dev.vercel.app/serverless/observability/analyze-hosts

note that the base URL changes when new commits are pushed.

[github-actions]

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[mergify]

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

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[elasticdocs]

🚀 Built co-preview-docs successfully!

• https://docs-elastic-odt7mapih-elastic-dev.vercel.app

• Built with commit d4e65e2

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[mdbirnstiehl]

LGTM!