[MNGSITE-544] Outline Plugin configuration inheritance/merging

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNGSITE-544](https://issues.apache.org/jira/browse/MNGSITE-544?redirect=false)** and commented

Determining effective plugin configurations is even more complex than other parts of the POM as there are multiple different sources where they could be defined:

1. Parent or Current POM
2. Plugin Mgmt or Plugins
3. Global Plugin Configuration or Execution ID specific configuration

The order of calculating the effective plugin configuration should be outlined in https://maven.apache.org/guides/mini/guide-configuring-plugins.html

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No further details from [MNGSITE-544](https://issues.apache.org/jira/browse/MNGSITE-544?redirect=false)