Document how to migrate plugins for release 5.7

The release 5.7 brings many backwards incompatible changes to the plugin system, starting with missing ParaViewConfig.cmake and UseParaView.cmake files, which were used by plugins in older versions. Although there seems to be documentation which is up-to-date, there is no indication of the recent changes. Moreover, an older version of the page does not seem to be available, so users can't even try to figure out the differences themselves.

Please provide a proper migration guide.