[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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[approksiu]

LGTM!

[benironside]

Nice work, just left a few suggestions for your consideration!

[joepeeples]

A few more line edits, thanks! Ping again for approval!

[nastasha-solomon]

Verified that this feature appears in the UI on prod and works as intended.