doc: document how to configure the test in playwright.config.ts

## Description 

- Some users might not be able to run the R command here, so we need to point them how to configure it themselves 

```
  webServer: {
    command: 'R -e "setwd(here::here());options(shiny.port = 3000); shiny::runApp()"',
    url: 'http://127.0.0.1:3000',
    reuseExistingServer: !process.env.CI,
  },
```