Rewrite README to highlight value over manual normalizes

[cmer]

Summary

• Reframes the README around the core question: why use this gem instead of writing normalizes lambdas by hand?
• Adds a side-by-side code comparison and a bullet list of concrete wins (global defaults, schema-aware bulk application, composable rules, smart nullify, multibyte-safe whitespace).
• Promotes the "sane defaults" initializer pattern, replaces the configuration option list with a scannable table (and adds the missing only/except rows), and moves the upstream-normalizes behavior caveats below the value pitch.

Test plan

• Skim the rendered README on the PR page to confirm formatting and the table render correctly.

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