docs(ClassicalMechanics): expand the Euler-Lagrange module overview

[sankalpsthakur]

Expands the Euler-Lagrange module documentation: the mathematical setting (trajectories in a complete real inner-product space, the action functional), the main definitions and results (eulerLagrangeOp, eulerLagrangeOp_eq, eulerLagrangeOp_zero, euler_lagrange_varGradient), and current scope (smooth data, Hilbert-space-valued trajectories; system-specific applications live in their own modules).

Documentation only — no declarations, proofs, or signatures changed.

AI/LLM disclosure

AI coding tools were used to help draft this documentation. I reviewed the complete change for accuracy before submitting.

[github-actions]

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