Add documentation for plotting and data visualization methods

Add documentation for methods of data access with cdippy at to [this page](https://github.com/cdipsw/CDIPpy/blob/main/docs/user_guide/plotting.md).

See:
- [current documentation](https://cdipsw.github.io/CDIPpy/user_guide/plotting/)
- [contributing documentation](https://cdipsw.github.io/CDIPpy/dev_guide/contribute/#documenting)