Ozone: generate optional, version specific documentation during the build

HDFS-12664 susggested a new way to include documentation in the KSM web ui.

This patch modifies the build lifecycle to automatically generate the documentation if hugo is on the PATH. If hugo is not there the documentation won't be generated and it won't be displayed (see HDFS-12661)

To test: Apply this patch on top of HDFS-12664 do a full build and check the KSM webui.