docs(Electromagnetism): document the three-dimensional Maxwell equations module

[sankalpsthakur]

Documents the three-dimensional Maxwell equations module: lists the four results it proves (gaussLawElectric, gaussLawMagnetic, ampereLaw, faradayLaw), explains how they relate to the covariant IsExtrema formulation (the two inhomogeneous equations come from the extremality condition, the two homogeneous ones follow directly from the potential definitions), and notes the current scope (pointwise differential form in free space; integral formulations, boundary conditions, and material media are out of scope).

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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