docs: add typed custom produce function

[oriondean]

Resolves #898

A small addition to docs demonstrating TypeScript and React typed custom prouce functions.

[mweststrate]

First of all, thanks for adding this pattern! Secondly: sorry for the late response.

Would you mind moving this pattern to typescript.md instead (https://immerjs.github.io/immer/typescript)?

Feel free to add a link to produce.md though to highlight that advanced typing scenarios are covered there, that might now not immediately obvious indeed.