[DOCS] Add description for untracked alert status

[lcawl]

Summary

Relates to #164059, #159462

This PR updates https://www.elastic.co/guide/en/kibana/current/create-and-manage-rules.html as follows:

• Adds untracked to the list of valid alert statuses
• Clarifies how to mute and untrack alerts
• Refreshes a screenshot to show the latest layout of the alerts table
• Moves information about muting rules together under the same section and deletes a redundant screenshot
• Moves the "importing and exporting rules" section to the end of the page, since it's a less common task

It also updates a screenshot in the index threshold rule example to show the updated layout for the alerts table.

Preview

https://kibana_bk_176974.docs-preview.app.elstc.co/guide/en/kibana/master/create-and-manage-rules.html

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[Zacqary]

LGTM

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