feat: support better port 0 user facing messages (#2859)

If a user chooses to use port 0, then the operating system will choose
one of the available ports automatically. This is very useful in cases
where you want to allow a `zola serve` for a broader user base which
might already bind a subset of their ports and if you don't know which
ones they might use.

This feature was already supported by zola, however, the printed output
just printed port 0 and didn't report which port the operating system
actually chose.

Example output of `zola serve -p 0` before the change

```
Web server is available at http://127.0.0.1:0 (bound to 127.0.0.1:0)
```

Example output of `zola serve -p 0` after the change

```
Web server is available at http://127.0.0.1:37327 (bound to 127.0.0.1:37327)
```

Author: RobWalt

src/cmd/serve.rs

@@ -543,7 +543,9 @@ pub fn serve(
 
                 println!(
                     "Web server is available at {} (bound to {})\n",
-                    &constructed_base_url, &bind_address
+                    &constructed_base_url
+                        .replace(&bind_address.to_string(), &server.local_addr().to_string()),
+                    &server.local_addr()
                 );
                 if open {
                     if let Err(err) = open::that(&constructed_base_url) {