Workload Protection reorg: Detect and Monitor content revisions

[drichards-87]

What does this PR do? What is the motivation?

This PR refines the Workload Protection Detect and Monitor documentation for clearer page structure, terminology, and navigation.

What changed	Why
Reworked the Agent Rules introduction and promoted its main topics to sibling sections	Clarifies how Agent rules, Agent events, actions, and backend detection rules relate without presenting navigation choices as sequential steps
Removed redundant introductory sentences and Overview headings from Content Packs, Variables and Actions, and the SecL guide	Starts each page directly with its substantive content
Replaced the question headings on Content Packs and folded the policy-page redirect into the preceding enablement step	Aligns the page with documentation style and keeps the procedure action-oriented
Corrected the SecL guide heading hierarchy	Makes expression concepts and the Agent/backend rule relationship appear at the appropriate levels
Standardized page and navigation titles for Agent Rules, Policy Management, Detection and Finding Rules, Detection Rules, and Finding Rules	Keeps page titles and navigation labels consistent

This changes eight files. No pages, URLs, or documented capabilities were added or removed.

Merge readiness

• Ready for merge

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Additional notes

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Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/agent_rules/
• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/agent_rules/policy_management
• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/agent_rules/secl_guide
• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/agent_rules/variables_and_actions
• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/content_packs
• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/detection_and_finding_rules/
• https://docs-staging.datadoghq.com/deforest/wp-detect-monitor-content-review/security/workload_protection/detect_and_monitor/detection_and_finding_rules/detection_rules