[DOCS-3411] Add new security notification and variables docs

[maycmlee]

What does this PR do?

• Adds new Security Notifications doc and makes it a parent in the nav.
• Adds new notification Variables doc under Notifications.
• Moves Rules doc under new Notifications parent in nav.
• Adds links to the new docs.
• Updates links to the Rules doc's new location.

Motivation

DOCS-3411

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/notifications/
• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/notifications/variables

Renamed files

• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/notifications/rules

Modified Files

• https://docs-staging.datadoghq.com/may/add-notifications-vars/account_management/billing/usage_details
• https://docs-staging.datadoghq.com/may/add-notifications-vars/getting_started/integrations/aws
• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/application_security/custom_rules
• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/cloud_siem/log_detection_rules
• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/cloud_workload_security/workload_security_rules
• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/detection_rules/
• https://docs-staging.datadoghq.com/may/add-notifications-vars/security_platform/security_signal_management/
• https://docs-staging.datadoghq.com/may/add-notifications-vars/tracing/trace_collection/library_config/go

[maycmlee]

Thanks @alai97 for the feedback! I made all the changes, could you re-review?