Sling API docs at javadoc.io
javadocs.io provides archived javadocs for all Sling modules (and actually for all open source software hosted at Maven Central).
To get the javadocs of a particular module use an URL like http://javadoc.io/doc/{org}/{artifact}/{version}
, like
for example http://www.javadoc.io/doc/org.apache.sling/org.apache.sling.api/2.9.0