docs(Electromagnetism): expand the foundational module overview

[sankalpsthakur]

Expands the Electromagnetism foundational module from a two-line stub noting it "is old and parts of it will soon be replaced" into proper module documentation: what the module provides (field/potential/density types, EM constants), current status relative to the more developed Kinematics/Dynamics/Distributional/ThreeDimension/Vacuum subdirectories, and future-work notes (generalizing charge/current density to signed measures, consolidating legacy interfaces as specialized APIs stabilize). Also tightens a few docstrings (spacing, minor grammar) on the existing declarations without changing them.

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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