Add opus 1.3.1 documentation.

This was built today from the 1.3.1 tag, so it suffers from
the navbar issue fixed on the development branch.
1 job for opus-1.3.1-doc in 1 minute and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #10493
docker
build

00:01:32