Documentation module build can allow content to be duplicated

Because of the way the content directories are copied into a common directory, sections can be duplicated if a maven `clean` step is omitted between switching versions or during a release build. Adding a version number to the directories would cause only the correct files to be used each time.

Steps to Reproduce

1. check out a release branch
2. build with `mvn install`
3. check out master branch
4. build with `mvn install`
(build will likely fail unit tests if branches have diverged, but html files can be opened and checked)