Blink provider (API) URL should be more discoverable in educational materials

[cbosborn]

Even with the educational material in the starter front end, it is still hard to find out what the URL for the blink provider (API) is, i.e. in case I want to construct transactions programmatically by calling the POST API URL.

An easy solution would simply be to state this in the educational material in the client app, in the ⚙️ backend section: <base-url>/api/actions/donate-sol/