update docs

docs/Makefile

docs/source/api/codegen.rst

docs/source/api/field.rst

docs/source/api/jit.rst

docs/source/api/sympyextensions.rst

docs/source/backend/index.rst

docs/source/backend/jit.rst

docs/source/index.rst

docs/source/reference/api/index.rst

docs/source/reference/gpu_kernels.md

docs/source/reference/kernelcreation.md

src/pystencils/jit/__init__.py