[Security][9.1 & Serverless] Improvements to the gap remediation feature

[nastasha-solomon]

Contributes to the following doc issues for 9.1 and Serverless:

• [Internal]: Gaps became GA #2111
• [Internal]: Rule gaps callout and gaps overview panel #1887
• [REQUEST]: Add docs for the new bulk gap filling feature #1435

Corresponding 8.19 docs: elastic/security-docs#6935

Previews:

• Manage detection rules | Fill gaps for multiple rules - New section
• Rule Monitoring tab - Reformatted docs about the panel above the Rules table to show how the UI differs between 9.0 and 9.1.
• Gaps table - Added tags to show that the Gaps table is beta in 9.0 and GA in 9.1. Also included a note that lets users know how to bulk-fill gaps for multiple rules.

[github-actions]

🔍 Preview links for changed docs

• solutions/security/detect-and-alert/manage-detection-rules.md
• solutions/security/detect-and-alert/monitor-rule-executions.md

[mdbirnstiehl]

LGTM!

[denar50]

LGTM!