docs: add legacy doc aliases for engine and front-matter refs (#1626)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jamesadevine <4742697+jamesadevine@users.noreply.github.com>

Author: Copilot

docs/engines.md

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+# Moved: `docs/engine.md`
+
+This page moved to [`docs/engine.md`](engine.md).
+
+If you reached this page from an older link, use the canonical engine reference:
+
+- [`docs/engine.md`](engine.md)

docs/reference/front-matter.md

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+# Moved: `docs/front-matter.md`
+
+This page moved to [`docs/front-matter.md`](../front-matter.md).
+
+If you reached this page from an older link, use the canonical front-matter reference:
+
+- [`docs/front-matter.md`](../front-matter.md)