[DOCS] add plot_pull documentation

[DraTeots]

Hello! We are trying to use plot_pull in our analyses but unfortunately just providing a PDF function without other fit parameters like default values makes it rather not working than working. At the same time documentation about plot_pull currently looks like this

How do you want to improve the docs

0. How the histogram is fitted (what method and engine is behind).
1. How one provides initial values?
2. How one retrieves obtained fit parameters?
3. Can one lock values/refit?

Even the only example is not documented.

# plot pull
h.project("W").plot_pull(
    pdf,
    eb_ecolor="green",
    eb_mfc="green",
    eb_mec="green",
    eb_fmt="o",
    eb_ms=5,
    fp_color="lightseagreen",
    pp_color="darkgreen",
    pp_alpha=0.4,
    pp_ec=None,
    bar_color="darkgreen",
)
plt.show()

One may guess what those mean in general while pp_ec is really intriguing!

P.S. Please, documentation is needed more than features for such library.

[LovelyBuggies]

Thank you for reporting this issue, we will add the docs before the next release. Currently, plot_pull is calling plot_ratiolike which uses scipy.optimize fitter.