Add note on how to run the `ui` project in the documentation

**Is your enhancement request related to a problem? Please describe.**
The docs at [this article](https://blog.heroku.com/building-a-monorepo-with-yarn-2) explain how to build a monorepo on Heroku, but the `start` process only starts the backend.

**Describe the solution you'd like**
Add a note on how to start the frontend, or how to link the backend to see it.

**Describe alternatives you've considered**
I'm using https://medium.com/inato/how-to-setup-heroku-with-yarn-workspaces-d8eac0db0256 this article that requires two separate Heroku instances, each starting a dyno.

Start with the linked Heroku monorepo article and inspect the existing documentation and start process for the ui project. Document how to start the frontend or connect it to the backend, and consider the separate-Heroku-instance approach described in the alternative article. Done means the documentation clearly explains how to run and view the frontend.

Written by the indexing model from the issue text.