Split maven plugin and site module in 2 separated submodules

[abelsromero]

Description

In order to better evolve the tools we need to split each set of features on its own modules.
There's quite confusions about the subset of features that we explain much better since the antora docs migration but spliting the code seems the logical step.

Other considerations

• As submodules under same parent we will still release two at the same time even if there are no changes in one of them.
• As a nice extra, users using the plugin won't be downloading doxia libraries which they don't need.
• Requisit for Create maven-site/doxia compliant module #571