[!32] Fix broken doxygen links in manual.
Merge branch 'bugfix/fix-broken-doxygen-links' into 'master' ref:core/dune-grid-howto The links now point to the current unstable version of the documentation. I changed the link Iterator to EntityIterator because from the context I assume that's what was meant. Also, there is a paragraph left about EntityPointer, which are no longer there (therefore that link is still broken even in this MR). See merge request [!32] [!32]: gitlab.dune-project.org/core/dune-grid-howto/merge_requests/32