Add mkdocs documentation

CMakeLists.txt

doc/CMakeLists.txt

doc/Installation.md

doc/Mainpage.md

doc/doxygen/CMakeLists.txt

doc/doxygen/Doxylocal

docs/Doxyfile

docs/Doxylayout.xml

docs/css/extra.css

docs/getting-started.md

docs/howto.md

docs/index.md

docs/reference/DOFVector.md

docs/reference/GridFunctions.md

docs/reference/MatVecBase.md

docs/reference/Operators.md

docs/reference/Problem.md

docs/tutorials/grids-and-discretefunctions.md

docs/tutorials/tutorials.md

mkdocs.yml