[8.18] [8.18] Rule gaps and manual rule runs (backport #6649)

[mergify]

Addresses #6493, elastic/docs-content#287, and elastic/docs-content#888 by providing 8.18 docs for rule gaps and updating docs for manual runs. Note that I also shifted some manual run content around for better flow or to provide more context where it was lacking.

Twin 9.0 and Serverless PR: elastic/docs-content#892

Previews:

• Rule Monitoring tab: Refreshed the intro para and screenshot to show that you can find gap details on the Rule Monitoring tab. Also added a short para to the end of the section to elaborate.
• Execution results tab: Since this section has evolved into an explanation of what's in the Execution results tab on the rule details page, I changed the section name from "Execution results" to "Execution results tab". I also moved content about the Execution log into its own sub-section titled "Execution log table".
  • Gaps table: New section that explains how to use the Gaps table to monitor and fill gaps.
  • Manual runs table: Made a few changes:
    • Removed instructions for accessing the Manual runs table. Since they were generally applicable to all of the tables within the Execution results tab, I provided general guidance to the Manual runs table (and all other tables on the tab) in the intro para for the "Execution results tab" section.
    • Elaborated on what manual runs were and linked to the instructions for starting manual runs to allow users a way to quickly access those steps.
    • Combined related actions and ideas in the list.
• Manage detection rules | Run rules manually: Made the following changes:
  • Removed the beta tag since manual runs is GA'ing in 8.18. Also refreshed the image to show that the table no longer had a pre-release label.
  • Refreshed intro so it shows that you can manually run rules to fill gaps.
  • Moved a bit of content around at the end for better flow. Also updated the note to include the alert suppression known issue.

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  This is an automatic backport of pull request [8.18] Rule gaps and manual rule runs #6649 done by [Mergify](https://mergify.com).

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