1. Removing "welcome" page. We no longer need it as we have updated the
   online website for ParaView docs.
2. Make all generated doc be generated in the final location rather than
   generating them in a temporary location and then moving them.
3. Remove `PARAVIEW_WWW_DIR`. This variable was no longer used for
   anything except generated docs. Adding
   `PARAVIEW_GENERATED_DOCUMENTATION_OUTPUT_DIRECTORY` for the same
   instead.
4. Adding new target `ParaViewDoc-TGZ` to build documentations and
   generate a `ParaViewGeneratedDocs.tar.gz`.