docs: mention TypeScript generator options

[popsiclelmlm]

Related to expressjs/express#7231

Summary:

• clarify that express-generator creates a JavaScript application skeleton
• add a short TypeScript section to the generator page
• reuse the existing community caveat before linking to a community TypeScript generator

Tests:

• npm test -- --quiet en/starter/generator.md

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[popsiclelmlm]

Rebased this on the current main branch after the site content migration and moved the TypeScript generator note into the versioned MDX starter pages.\n\nLocal checks:\n\n- npm run format:check -- src/content/docs/en/4x/starter/generator.mdx src/content/docs/en/5x/starter/generator.mdx\n- npm run build