Add instructions for running the app locally in production mode

The [docs](https://github.com/googlemaps/js-route-optimization-app/blob/bab7a257ce1187cc7baba945cf475b3569c35f84/docs/development.md?plain=1#L223-L233) make it clear how to run this app locally in "watch" mode.

It would be helpful if there were also instructions on how to run this app locally, but in a more optimized mode (commonly referred to as "production" or "preview" mode). Watch mode is great for when you're actively hacking on the code. But when you're serving the app over a network it creates unnecessary overhead.

I tried to `npm run build` in both the frontend/backend I kept facing issues after running the built files where the `MAPS_API_KEY` and `GOOGLE_APPLICATION_CREDENTIALS` were not being recognized (the credentials themselves are valid as they work fine when running in watch mode). Should I have used the docker(-compose) files instead?

My use case is that I want to build the app, run it locally, then serve it with a [reverse proxy over an internal network](https://github.com/tailscale/caddy-tailscale).