Call out that jobs config should contain at least one visible job

Problem to solve

• What product or feature(s) affected?
  • GitLab CI/CD Pipeline Configuration Reference
• What docs or doc section affected? Include links or paths.
  • https://docs.gitlab.com/ee/ci/yaml/#introduction
• Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs?
  • Minimum requirement for a stage job needs specifying

Further details

Documentation does not state that a pipeline yaml file must contain at least one visible job.

As an example, the following causes a pipeline lint error (and the pipeline to fail if run):

stages:
  - build
  - test
  - deploy
  
.build-it:
  stage: build
  script: echo "gradle build"

.test-it:
  stage: test
  script: echo "gradle test"
  
.deploy-it:
  stage: deploy
  script: echo "gradle deploy"

Error message:

Status: syntax is incorrect 

Error: jobs config should contain at least one visible job

Proposal

Update the Introduction section to specify:

There must be at least one visible job, otherwise an error will occur. See Hidden keys for details on hiding a job.

Who can address the issue

Documentation team

Other links/references

N/A