Improve documentation for how include works in context of extends

The documentation for extends is missing an example of using it with include, which was an important part of why it was built.

• Documentation: https://docs.gitlab.com/ee/ci/yaml/#extends
• Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/6136

The documentation should describe a use case something like the following as an example of how to include using extends:

included.yml

.template: 
  script:
    - echo Hello!

.gitlab-ci.yml

include: included.yml

useTemplate: 
  image: alpine
  extends: .template

This will run a job in the .gitlab-ci.yml called useTemplate that runs echo Hello! as defined in the template and uses the alpine Docker image as defined in the local job.

cc @markpundsack