extended backend documentation

docs/source/backend/index.rst

docs/source/backend/iteration_space.rst

docs/source/backend/kernelcreation.rst

docs/source/backend/platforms.rst

docs/source/backend/translation.rst

src/pystencils/backend/kernelcreation/__init__.py

src/pystencils/backend/kernelcreation/analysis.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelcreation/iteration_space.py