Add documentation for transformations module. Fix some doc comments.

docs/source/backend/index.rst

docs/source/backend/transformations.rst

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/transformations/__init__.py

...ils/backend/transformations/hoist_loop_invariant_decls.py

src/pystencils/backend/transformations/reshape_loops.py