Add style guide for YAML reference

What does this MR do?

As part of our effort to improve the YAML reference, we need a simple style that engineers can easily copy and reused throughout the reference. We also want a style that helps make the page more scannable, which improves the usability for our users. This style is based on the final style used in !58880 (merged), and can be seen for only and except right now.

The style should make the page drier, and much more similar to an API reference, with only the basic details about the keyword syntax and use. Extra details, use cases, and complicated examples should be documented separately. For example, only/except now has a separate page with more use cases and examples: https://docs.gitlab.com/ee/ci/jobs/job_control.html#onlyrefs--exceptrefs-examples

Related issues

Resolves #324798 (closed)

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