Document plot handlers.

[pfebrer]

Methods like Hamiltonian.plot.pdos() are not documented anywhere, and the only way of getting information about them is if your editor can show you the arguments and documentation of the function.

I want to document them, but I don't know how exactly. I have thought of modifying the class template for autosummary (https://github.com/zerothi/sisl/blame/main/docs/_templates/autosummary/class.rst) to check for the plot attribute and then add some documentation, but I don't know exactly how or if this is the best approach.