Add documentation for Kafka engine virtual columns

Added documentation for Kafka engine virtual columns, including built-in and extra columns for parse-error streaming.

Author: lesandie

content/en/altinity-kb-integrations/altinity-kb-kafka/altinity-kb-kafka-virtual-columns.md

@@ -0,0 +1,40 @@
+---
+title: "Kafka engine Virtual columns"
+linkTitle: "Kafka virtual columns"
+weight: 100
+description: >-
+     Kafka virtual columns
+---
+
+## Kafka engine virtual columns (built-in)
+
+[From the Kafka engine docs](https://clickhouse.com/docs/engines/table-engines/integrations/kafka?utm_source=chatgpt.com#virtual-columns), the supported virtual columns are: 
+
+- `_topic` — Kafka topic (LowCardinality(String))
+
+- `_key` — message key (String)
+
+- `_offset` — message offset (UInt64)
+
+- `_timestamp` — message timestamp (Nullable(DateTime))
+
+- `_timestamp_ms` — timestamp with millisecond precision (Nullable(DateTime64(3)))
+
+- `_partition` — partition (UInt64)
+
+- `_headers.name` — header keys (Array(String))
+
+- `_headers.value` — header values (Array(String))
+
+Extra virtual columns when you enable parse-error streaming:
+
+If you set `kafka_handle_error_mode='stream'`, ClickHouse adds: 
+
+- `_raw_message` — the raw message that failed to parse (String)
+
+- `_error` — the exception message from parsing failure (String)
+
+Note: `_raw_message` and `_error` are populated only when parsing fails; otherwise they’re empty. 
+
+We can use these columns in a materialized view like this for example:
+