Probing Metal Complexation, Structure, Ligand Lability and Dissociative Ligand‐Exchange Mechanism in the Slipped Triple‐Decker Complexes [{(η5‐CpR)Co}2‐μ‐{η4:η4‐arene}] (R=Me5, 1,2,4‐tri‐tert‐butyl; arene=toluene, benzene)