[handbook]

Restructured handbook and handbook files as we discussed
it in the handbook meeting. This change is hardly reviewable
so please compile and check whether you miss something in the
new version of the handbook or the structure appears weird.

The models have been removed, because they are just a copy from the
doxygen description and we decided to refer to the doxygen homepage
for a detailed description (which also upgrades doxygen).



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15030 2fb0f335-1f38-0410-981e-8018bf24f1b0