Add documentation on Gaussian quadrature

[marco-2023]

Added a Jupyter notebook explaining Gaussian Quadrature.

Missing: We can make links from here to the different notebooks of the quadrature rules implemented in GRID

[FarnazH]

@marco-2023, I am wondering whether the content of this notebook should be added at the beginning of the https://grid.qcdevs.org/pyapi/grid.onedgrid.html module (as a string). This will be like the algorithms described at the beginning of https://grid.qcdevs.org/pyapi/grid.periodicgrid.html module. Feel free to let me know what you think.