[Doxylocal]: added units package for improved latex support

[mainpage.txt]: added links to Modules overview page and to
parameterlist.txt

Reviewed by Thomas


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15187 2fb0f335-1f38-0410-981e-8018bf24f1b0