symbolic extensions export module + documentation

docs/source/api/index.rst

docs/source/api/symbolic_language/astnodes.rst

docs/source/api/symbolic_language/field.rst

docs/source/api/symbolic_language/index.rst

docs/source/api/symbolic_language/sympyextensions.rst

docs/source/api/sympyextensions/index.rst

src/pystencils/symb.py

src/pystencils/sympyextensions/integer_functions.py