small mdspan refactor. Added install instructions and usage primer.

docs/index.md

mkdocs.yml

src/pystencilssfg/source_concepts/cpp/__init__.py

src/pystencilssfg/source_concepts/cpp/std_mdspan.py

tests/cmake_integration/kernels.py

tests/mdspan/Makefile

tests/mdspan/kernels.py

tests/mdspan/main.cpp