Redirect browser accessing to PD to Dashboard

## Feature Request

### Describe your feature request related problem

I want to access dashboard, but I don't know the exact URL. So I use tiup to display the cluster, it shows a PD with UI mark. So I guess dashboard should be hosted in the address. But no matter connecting 2379 or 2380 will get a 404 status code.

### Describe the feature you'd like

When accessing PD using browser without extra path, it can redirect the request to dashboard instead. PD can use user agent to tell if it's a request made in browser.

### Describe alternatives you've considered

Display the full dashboard url in tiup display. Show full url can be more accurate, and if dashboard is moved outside of PD, the entry point can be reused. But redirecting seems more convenient.

### Teachability, Documentation, Adoption, Migration Strategy