Revise cdp_ex_optgrowth_jl

[oyamad]

• Add explanations on the model formulation and the API.
• Revise the content:
  • Add explanation texts aligned with the QuantEcon lecture.
  • Wrap model parameters in an OptimalGrowthModel function returning a NamedTuple (for type stability).
  • Move the "A First Test" section (low-level interface demo) to the end.

[Copilot]

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