Docathon! Docs for discrete pulse functions

[DanPuzzuoli]

Closes #3392

Summary

Adding math expressions for the functions being sampled in discrete.py.

Details and comments

1. Updated doc strings in discrete.py to include math expressions, along with a couple of jupyter-execute sections for plotting pulses with a simple shape but whose mathematical expression may not be immediately recognizable (or at least weren't to me).
2. Code provided by @SooluThomas: Updated pulse.rst and pulse/__init__.py to include a section in the sphinx pulse toc called Pulse Library, which links to the discrete module.

[taalexander]

Looks great, just some small changes recommended.

[SooluThomas]

Thank you @DanPuzzuoli

[lcapelluto]

This is wonderful!! Looks so good now when rendered.

I had a minor question about a factor of two, pretty sure we include it. Here is what I mean, from continuous:

def gaussian(times: np.ndarray, amp: complex, center: float, sigma: float,
             zeroed_width: Optional[float] = None, rescale_amp: bool = False,
             ret_x: bool = False) -> Union[np.ndarray, Tuple[np.ndarray, np.ndarray]]:
    times = np.asarray(times, dtype=np.complex_)
    x = (times-center)/sigma
    gauss = amp*np.exp(-x**2/2).astype(np.complex_)

[DanPuzzuoli]

Nice catch @lcapelluto, just committed

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