docs(r/adbcflightsql): Add better example for running the FlightSQL driver from R

The example containers for FlightSQL have come a long way since first writing the documentation for the driver. We should add an example to the readme!

https://github.com/voltrondata/flight-sql-server-example

Note that the example has TLS enabled by default with a self-signed certificate, so you will also need:

```r
adbc_database_init(..., "adbc.flight.sql.client_option.tls_skip_verify" = "true")
```