Redirect your.project.site/api to javadoc.io API docs

[armanbilge]

https://typelevel.org/cats/api/ is easy to remember and an eager suggestion by my browser, but nowadays I really want to go to https://www.javadoc.io/doc/org.typelevel/cats-docs_2.12/latest/

So seems like we should use the meta-refresh trick to get an auto-redirect there and integrate this into the site plugin.

Annoyingly this could be a breaking change if someone was actually putting something at /api. But I think Laika would give priority to that. Also I really want this so I might look the other way 😬