Audit and improve/expand templates for CI/CD

Problem to solve

It is difficult to learn the features of GitLab CI/CD and how to configure the CI YML from documentation alone. Working examples that are well-documented are very helpful to achieve this understanding and get started quickly and successfully.

Further details

We have https://gitlab.com/gitlab-examples/ with working examples, though we don't annotate the YML and in some cases, the pipelines are not successful; and we have https://docs.gitlab.com/ee/ci/examples/ which are less helpful without the context of a full working project.

Proposal

Audit and align examples across https://gitlab.com/gitlab-examples/ and https://docs.gitlab.com/ee/ci/examples/, ensuring that each is a working example with annotated CI YML and it is easy to understand how it is configured and why.

Who can address the issue

A GitLab engineer and technical writer

Other links/references

Inspired by &2072 (comment 233510143)