[release] Improved documentation of the parallel section.

VariableSizeCommunicator now appears in doxyen and the parallel
index sets are a subsection of the parallel section.
[release,cmake] Install data files to package specific datadir like with automake

E.g. the files in am are now installed to share/am/dune-module instead of share/am.
[release,buildsystem] Fixes list of files to install.
[release,automake] Fixes occurences of dune-istl in install directories.

When moving the documentation of the parallel index sets from dune-istl
to dune-common, we forgot to adjust the installation directories accordingly.
This patch fixes this situation.
[release,automake] Fixes installation of files below am.

The am_DATA variable was set to EXTRA_DIST which contained
CMakeLists.txt. Therfore CMakeLists.txt was installed.
This patch fixes this by listing files to install in am_DATA
and using EXTRA_DIST only for those files that are otherwise
not distributed. Suprisingly, some files from am_DATA have to
be listed there, too.