Enable Threat Monitoring page by default

What does this MR do?

This enables the Threat Monitoring feature by default (and can be disabled by explicitly disabling the threat_monitoring feature flag).

This also adds some basic documentation for Threat Monitoring.

Part of WAF statistics reporting.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers