[#8]:svarga:docs, add usage examples

[steven-varga]

Summary

• docs/types.md — new type-selection guide: decision matrix + one working example per representation (BID, fixed, BCD, scaled)
• docs/scaled.md — replaced stale dev-era spec (described wrong API with kind field and make_zero()/make_inf() helpers that don't exist) with accurate math::scaled::decimal_t documentation

Test plan

• mkdocs serve renders both pages without errors
• All code snippets in docs/types.md compile against current staging headers
• docs/scaled.md API matches include/decimal/scaled.hpp (no phantom fields or helpers)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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