doc/.gitlab-ci.yml: deploy_doc should run when build_doc was run

see #4148.
5 jobs for !597 with doc/prod in 1 minute and 18 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Tgz Release
passed #525836
doc
build_doc

00:01:10

passed #525807
doc
build_doc

00:00:57

 
  Deploy Prod
passed #525850
allowed to fail manual
deploy_doc

00:00:07

passed #525827
allowed to fail manual
deploy_doc

00:00:09

passed #525808
allowed to fail manual
deploy_doc

00:00:07