docs: improve Tensor Train learning flow

[0xC000005]

Summary

• add a TT core-chain mental model to explain ranks and storage without requiring readers to start from the full formula
• make the first TT quick start complete: build, evaluate, compare exact value, print ranks, build evaluations, and compression
• add an explicit TT-Cross validation checklist for held-out points, rank caps, node tuning, and seed sensitivity
• add a troubleshooting table for common TT-Cross, dense-method, ErrorEstimate(), and finite-difference derivative symptoms

Refs #185. Complements #186, which handles the Slider/example portion.

Validation

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[0xC000005]

Superseded by #188, which combines the Slider and Tensor Train documentation updates with the 0.13.3 version bump and changelog entry.