docs: explain Slider and Tensor Train mental models

[0xC000005]

Summary

• add a worked Slider mental-model example for sin(x1) + cos(x2) + tan(x3)
• explain the Slider pivot, singleton slide grids, evaluation formula, and dense-vs-slider source-call count
• add the matching Tensor Train mental-model example showing the dense sampled tensor, TT cores, TT-SVD vs TT-Cross construction, and evaluation as basis-vector contractions through the core chain

No version bump: this is documentation-only and should refresh the docs site after merge.

Validation

• git diff --check
• docfx docs/docfx.json

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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