docs update config ref to include example using YAML anchors for base-variables

What does this MR do?

As per https://forum.gitlab.com/t/stop-environment-git-strategy-when-we-depend-on-global-variables/31700, in the stop environment job documentation example, it was unclear that the example had the adverse affect of wiping (overwriting) all base/global variables.

This is here: https://docs.gitlab.com/ee/ci/yaml/#environmentaction

This change denotes this fact, and adds an example of how to use YAML anchors to pull them in AND still set the GIT_STRATEGY to none.

Other technical references in case reviewers are unfamiliar:

• https://docs.gitlab.com/ee/ci/yaml/#git-strategy
• https://docs.gitlab.com/ee/ci/yaml/#anchors

EDIT: per review feedback, we ended up putting the generic method in anchors section, and just referencing it from the environments section

Related issues

N/A

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.