Add link to .scala-steward.conf documentation to PRs

Recent PRs enhanced the capabilities of repo specific configuration via `.scala-steward.conf` (#1274, #1254, #1330, #1336). To increase the discoverability of these features, we could link to the [`.scala-steward.conf` documentation](https://github.com/fthomas/scala-steward/blob/master/docs/repo-specific-configuration.md) in PR bodies.