docs: Only build service docs if service build is enabled

Otherwise, `install` target fails when service build is disabled.
4 jobs for master in 10 minutes and 37 seconds (queued for 7 minutes and 33 seconds)
Status Job ID Name Coverage
  Test
passed #167261
ubuntu-16.04

00:02:47

passed #167262
ubuntu-17.10

00:02:36

passed #167263
ubuntu-18.04

00:02:46

passed #167264
ubuntu-18.04-no-backend

00:02:27