docs: Only build service docs if service build is enabled

Otherwise, `install` target fails when service build is disabled.
4 jobs for separate-xml in 11 minutes and 16 seconds (queued for 35 minutes and 28 seconds)
Status Job ID Name Coverage
  Test
passed #167142
ubuntu-16.04

00:02:44

passed #167143
ubuntu-17.10

00:02:42

passed #167144
ubuntu-18.04

00:02:26

passed #167145
ubuntu-18.04-no-backend

00:03:22