docs: clarify template engine adapters

[vibhor-aggr]

Summary

• Clarify that generator-created apps already get template engine settings, while non-generator apps must configure them explicitly.
• Replace the raw Handlebars package reference with Handlebars-compatible engine wording.
• Note that Handlebars needs an Express-compatible view engine or adapter rather than the raw handlebars package alone.

Fixes #2128.

Verification

• npx prettier --check src/content/docs/en/4x/guide/using-template-engines.mdx src/content/docs/en/5x/guide/using-template-engines.mdx
• npx cspell src/content/docs/en/4x/guide/using-template-engines.mdx src/content/docs/en/5x/guide/using-template-engines.mdx
• git diff --check
• npm run lint
• npm run build

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