updating download/index.md should be automated

currently the script just says:

`src/main/scala/MakeReleaseNotes.scala:      println(s"# don't forget to\n${scala.util.Properties.envOrElse("EDITOR", "mate")} ../scala-lang/download/index.md ../scala-lang/documentation/api.md ../scala-lang/_config.yml")`

but the manual changes you need to make to download/index.md are annoying