docs(ClassicalMechanics): expand the module overview

[sankalpsthakur]

Expands the Classical Mechanics module overview from a stub ("This file is currently a stub. Please feel free to contribute!") into a proper module documentation section following the library's # A, ## A.1/A.2/A.3 heading convention: scope of the module, current status (introduces no declarations itself), and future-work guidance for where shared vs. system-specific declarations belong.

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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[doxtor6]

Looks good. Again, It will be great if you can help to update the API-maps in the meanwhile. See details in https://github.com/jstoobysmith/PhyslibAITools

[jstoobysmith]

This is great! Many thanks for this first PR! Will merge this now