Add tutorial for picket-fence model Hamiltonian

[Prajjwal-21]

This PR adds a tutorial demonstrating how to construct and solve a simple model Hamiltonian using the ModelHamiltonian package.

The tutorial covers:

• constructing a model using HamHeisenberg
• generating zero-, one-, and two-body integrals
• solving the system using Full Configuration Interaction (FCI) with PySCF
• analyzing how the ground state energy scales with system size
• exploring the effect of interaction strength on the system

The goal of this tutorial is to provide a clear and practical introduction for new contributors to understand the workflow of the package.

It also highlights how the framework builds Hamiltonians in terms of fermionic operators through the generated integrals, connecting naturally to more general spin-based models.

Working on this tutorial also helped me better understand the internal structure of the package, especially in the context of potential extensions related to spin operator representations.

@RichRick1 I’ve opened this PR for the tutorial we discussed—looking forward to your feedback whenever you have time.