The old Zod 3 docs had a "Writing generic functions" section that showed users how to accept Zod schemas as function parameters. The Zod 4 docs have equivalent (and better) content in /library-authors, but that page is explicitly scoped to library authors — a general user wouldn't think to look there.

This adds a concise "Writing generic functions" section to the basics page covering:

• The correct generic pattern (T extends z.ZodType not z.ZodType<T>)
• Extracting the inferred type with z.output<T>
• Constraining allowable input types

A cross-reference to /library-authors points users to the full treatment for advanced patterns (Zod Mini, Zod 3/4 dual-support).

Closes #6060

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