Improve README.md

In particular, this imports a few bits from the old dune-functions page in dune-website. In the future, the website will show the README.md file directly.

enabled an automatic merge when the pipeline for bda0af09 succeeds

The CI tests fail because the test grid curved2d.msh is searched in dune-uggrid rather than dune-grid all of a sudden:

what():  Dune::IOError [read:/duneci/modules/dune-grid/dune/grid/io/file/gmshreader.hh:386]: Could not open /duneci/modules/dune-uggrid/doc/grids/gmsh/curved2d.msh

@simon.praetorius @santiago.ospina , could this be fallout from your recent build system changes?

mentioned in merge request fufem/dune-fufem!216 (merged)

mentioned in merge request fufem/dune-fufem!217 (merged)

mentioned in issue core/dune-common#369 (closed)

resolved all threads

mentioned in commit 4ee403a6

merged