Add Formisch example to forms guide

[Formisch](https://formisch.dev/react/guides/introduction/) is a modern headless form library that recently added support for React. It would be great to see an example of how to use Base UI forms with Formisch similar to the examples for [RHF](https://base-ui.com/react/handbook/forms#react-hook-form) and [TanStack Form](https://base-ui.com/react/handbook/forms#tanstack-form).

Thanks for the amazing library!