docs: add ReplacementTransformer reference

[locker95]

Summary

• Adds a reference page for the built-in ReplacementTransformer under site/content/en/docs/Reference/API/Transformers/, matching the format of other transformer docs (LabelTransformer, AnnotationsTransformer, etc.).
• Documents the replacements field and how to invoke the transformer via the Kustomization transformers field.
• Points readers to the existing replacements field documentation for the full schema and field behavior.

This addresses the missing entry called out in #5812 (other transformers are documented under this API reference section).

Test / Verification

• New page path: site/content/en/docs/Reference/API/Transformers/ReplacementTransformer.md
• Examples aligned with the built-in plugin tests in plugin/builtin/replacementtransformer/
• relref targets match existing Transformers index and Kustomization replacements docs
• Docs-only change; git diff --check clean

Fixes #5812

/kind documentation

[kubernetes-prow]

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• a3a6bf9 docs: add ReplacementTransformer reference

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This pull-request has been approved by: locker95
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