Hello,

this patch adds 'Doxia Sink API'[1] under the name
'maven-doxia-sink-api'.

The module contains a Java interface that is used by Doxia Core module.

This patch is the first in the future patch set based on my effort to
add Doxia modules.

The Doxia itself required for other useful Maven plugins.

- Artyom

References:
1. https://maven.apache.org/doxia/doxia/doxia-sink-api/