SymNMFk documentation

[MaksimEkin]

• SymNMFk missing documentation (https://github.com/lanl/T-ELF/blob/main/TELF/factorization/SymNMFk.py). Need to add documentation and re-generate Sphinx docs.
• Also, documentation website missing example usage for SymNMFk (https://github.com/lanl/T-ELF/blob/main/docs/source/SymNMFk.rst). See for example NMFk (https://github.com/lanl/T-ELF/blob/main/docs/source/NMFk.rst). Need to add example usage to documentation website for SymNMFk and regenerate Sphinx docs.