Prepared documention for upcoming 3.0.0 release

* Renamed folder ``user`` to ``2.0.x``.
* Created folder ``3.0.x`` and merged documentation of ``2.0.x`` and ``helium``.
* Removed references (i.e., :ref: links) because they cause ambiguity when the same
  label is defined twice (not surprisingly). Use :doc: instead, and always use relative
  paths!
* Created a symbolic link ``current-user-doc`` that should always point to the user
  documention for the current stable release. The intention here is to avoid absolute path
  to the user documentation from places such as tutorials. The symbolic link should point to
  folder ``3.0.x`` as soon as we release the Scala IDE v3.0.0.