Add monorepo example showing node/deno use in a front-end/backend project

Docs feedback for `/runtime/fundamentals/workspaces/`

> I could really use an example of setting up a monorepo with a frontend that relies on nodejs and a Deno backend, and code sharing between the two:
See https://stackoverflow.com/questions/79262566/monorepo-with-deno-and-node-js-vite-with-shared-packages-what-is-the-setup. Must be a common use case. Thanks for your work on Deno :-)