Improve MoleculeWidget API documentation in README

[Copilot]

The MoleculeWidget section of the README only listed method names in a single line with no parameter details, signal documentation, or usage guidance.

Changes

• Qt Signals — added a table documenting atomClicked(label: str) and bondClicked(label1: str, label2: str) with descriptions of when each fires
• Data methods — fully documented open_molecule(), grow_molecule(), and clear() including all parameter types (Atomtuple structure, cell tuple format, ADP dict layout, keep_view semantics)
• Display/view methods — individually documented show_adps, show_labels, show_hydrogens, show_round_bonds, set_bond_width (with valid range), setLabelFont, set_background_color, and reset_view
• Direct-data example — added a code snippet showing construction of Atomtuple objects, ADP dict, and signal connection without going through MoleculeLoader
• Typo fix — corrected "froma" → "from a" in the screenshot caption

from fastmolwidget import MoleculeWidget
from fastmolwidget.sdm import Atomtuple

mol = MoleculeWidget(parent=self)
atoms = [
    Atomtuple(label="C1", type="C", x=0.0,  y=0.0,  z=0.0,  part=0),
    Atomtuple(label="O1", type="O", x=1.22, y=0.0,  z=0.0,  part=0),
]
adps = {"C1": (0.02, 0.02, 0.02, 0.0, 0.0, 0.0)}
mol.open_molecule(atoms=atoms, cell=(5.0, 5.0, 5.0, 90.0, 90.0, 90.0), adps=adps)
mol.atomClicked.connect(lambda label: print(f"Selected: {label}"))