Connectors doc generation shuld be moved from `core` to `connectors` module

[valdar]

This https://github.com/apache/camel-kafka-connector/blob/master/core/pom.xml#L179-L191 should be moved from core to connectors module to ensure that it runs after connectors autogeneration.

To do so we must ensure that the plugin runs just one time and not once per connector see how it is done in the autogen plugin: https://github.com/apache/camel-kafka-connector/blob/master/tooling/camel-kafka-connector-generator-maven-plugin/src/main/java/org/apache/camel/kafkaconnector/maven/AbstractCamelKafkaConnectorMojo.java#L120-L123