Feature/move changes from webpage2modules

In light of the discussion in core/dune-common!334 (merged) I moved the few existing changelog entries for dune-grid to the CHANGELOG.md file in the releases/2.6 branch and modified the web page to link to the change log files of all modules. If we can agree on this I will add change log files to all core modules (empty for the master version and containing the existing entries in the releases/2.6 branch.