[Serverless] Manual rule run docs

[nastasha-solomon]

Contributes to #5264

Previews:

• Manage detection rules | Run rules manually: New section
• Made the following changes to the "Monitor and troubleshoot rule executions" page:
  • Execution results: Updated intro and added new list items to describe the new filter and toggle added to the Execution log table.
  • Added a new section to explain the new Manual runs table.
• Alert schema - Added a description for the new kibana.alert.intended_timestamp alert field.

Corresponding ESS PR: #5631

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[nastasha-solomon]

@nkhristinin and @approksiu - in the latest draft, I made the following changes:

• Added the limitations and best practices that we discussed in Slack. You can find the new additions in this section of the manual run docs.
• Doc'd the new kibana.alert.intended_timestamp alert field. You can find the description at the bottom of this table.

Please take a look at both updates when you have a chance. Thank you!

[approksiu]

LGTM!