Audit logger docs

[bryanherman]

Merge when the following Open WebUI PR gets merged: open-webui/open-webui#20114

This pull request adds documentation for two new environment variables that control where audit logs are output: to standard output and/or to a file. These options give users more flexibility in configuring audit log destinations.

Audit logging configuration:

• Added documentation for the AUDIT_ENABLE_STDOUT variable, allowing audit logs to be included in standard output when enabled.
• Added documentation for the AUDIT_ENABLE_FILE variable, allowing audit logs to be written to a file when enabled, depending on AUDIT_LOG_LEVEL.

[pr-validator-bot]

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