Include UML diagrams in javadocs [LUCENE-2296]

As Lucene source becomes more and more complex it would be helpful to provide a visual overview of Lucene classes in a form of UML class diagrams. These can be created automatically during the build process, using ApiViz (http://code.google.com/p/apiviz/).


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Migrated from [LUCENE-2296](https://issues.apache.org/jira/browse/LUCENE-2296) by Andrzej Bialecki (@sigram), 2 votes, updated May 09 2016
Environment:
```
Any environment supported by AT&T GraphViz, e.g. Linux, MS Windows, etc.
```

Attachments: [apiviz.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2296/apiviz.patch), [package-summary.png](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2296/package-summary.png) (versions: 2)

Start by reviewing the attached apiviz.patch and the build process that generates Lucene javadocs; the issue does not name current files or entry points. Check how ApiViz and AT&T GraphViz are expected to integrate, then verify that the build produces UML diagrams in the generated javadocs across a supported environment.

Written by the indexing model from the issue text.