Add docs for CLUSTER_SETTINGS_CHANGED and INDEX_SETTINGS_CHANGED audit categories

[cwperks]

Description

Adds public documentation for the new CLUSTER_SETTINGS_CHANGED and INDEX_SETTINGS_CHANGED audit log categories introduced in opensearch-project/security#6113.

Changes

• _security/audit-logs/index.md: Added the two new categories to the tracked events table and documented that they are disabled by default on the transport layer with instructions on how to enable them.
• _security/audit-logs/field-reference.md: Added field reference sections for both categories, including the audit_settings_changes array structure with sub-fields (setting, old_value, new_value, operation, scope). Updated the audit_category common attribute to list the new categories.

Issues Resolved

Documents opensearch-project/security#6113

Check List

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