Add link to "Manual stages and downstream dependencies in GitLab"

What does this MR do?

My MR is an attempt to add a reference from the "GitLab CI/CD \ Examples \ Implementing GitLab CI/CD" section to a tiny article explaining how to create a manual job with dependencies to another one.

Does this MR meet the acceptance criteria?

Conformity

Unapplicable

Availability and Testing

Unapplicable

Security

Unapplicable