Make inclusion of site docs optional for binary distribution

The fix for MPDIST-8 unfortunately has made "site" a prereq for dist:build-bin. OK, the fix does make the behaviour predictable, but I would not say "right". IMHO the full project site docs don't belong in a binary distribution at all. What you need there is exactly what was bundled in RC2: the jar, and the javadoc to use it. The site docs don't belong in the source distribution either, as it is so easy to generate them from the source.

Not to break anything depending on the current behaviour, the site plugin would need a property like maven.dist.bin.includeSite=true/false, the default being true.