Clean up documentation of Field and sympyextensions

docs/source/_templates/autosummary/sympy_class.rst

docs/source/reference/field.rst

docs/source/reference/symbolic_language.rst

docs/source/reference/sympyextensions.rst

src/pystencils/backend/ast/expressions.py

src/pystencils/field.py

src/pystencils/sympyextensions/math.py

src/pystencils/sympyextensions/typed_sympy.py