Publish API documentation

[Jasper-M]

Looks like the API for the most recent version didn't get published.

https://scala.github.io/scala-xml/api/1.3.0/scala/xml/

[SethTisue]

@ashawley should we ditch the GitHub.io based publication and just direct people to https://javadoc.io/doc/org.scala-lang.modules/scala-xml_2.13/1.3.0/index.html ? (did we discuss this before?)

[ashawley]

I've published 1.3.0 just for completeness. Yeah, feel free to ditch it. Just need to stop publishing the apiUrl in build.sbt to the scala.github.io address in the 2.0.0-Mx milestone series.

[SethTisue]

#472