Links to scaladoc broken when published

[richdougherty]

On this published page https://www.playframework.com/documentation/2.6.x/ScalaJsonCombinators the links to Reads and Writes are:

• https://www.playframework.com/documentation/2.6.x/api/libs/json/Reads.html
• https://www.playframework.com/documentation/2.6.x/api/libs/json/Writes.html

However they need to be changed to:

• https://www.playframework.com/documentation/2.6.x/api/scala/play/api/libs/json/Reads.html
• https://www.playframework.com/documentation/2.6.x/api/scala/play/api/libs/json/Writes.html

We need to check all of these links. (While we're at it we should probably check other bits of the Play documentation too to make sure our links work.)