Fix documentation links to external PDFs
We don't actually build the core modules anymore as part of the website rendering process, so we don't have current versions of all the different PDFs available anymore. So for now, this merge request improves things by
- Pointing people to the new Sphinx-based CMake docs instead of the old PDF and FAQ.
- Telling people where to find the other PDF-based documentation in the different core modules.
- Removing links to documentation of external modules (dune-fem, PDELab). I don't know how dune-fem handles things, but the PDELab Howto is actually pretty dead and superseded by the tutorials.
Maybe we should consider moving all that documentation from the current, TeX-based howtos to the website by adding a few markdown pages?
Fixes #22 (closed)