Enable Paraview embedded documentation

This option was set twice in Paraview superbuild project, once at ON, the second at OFF, overriding the first. This commit removes the second setting, enabling back the option.

If set to ON, plugins documentation is auto-generated from proxy XML files. This doc becomes available in LidarView Online Help, and can be accessed via the ? help button of the filter/source/etc in the Pipeline Browser.

Note that this option has nothing to do with code API documentation generation, and doxygen is not triggered via this command.