Docs - Add note about env slugs being immutable

What does this MR do?

Environment slugs are immutable.

If an environment is renamed, users might expect the slug to automatically rename as well.

This MR clarifies what happens here.

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• 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 and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.

changed milestone to %12.3

added documentation label

@eread Can you review please?

mentioned in issue #32526 (closed)

added missed:12.3 label

added Support Team Contributions label

added Technical Writing label

added backstage [DEPRECATED] label

changed milestone to %12.4

added devopsverify grouppipeline execution labels

assigned to @anton and @steveazz

added 1 commit

• 43e994c9 - Apply suggestion to doc/ci/environments.md

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approved this merge request

added devopsrelease [DEPRECATED] label and removed devopsverify label

removed grouppipeline execution label

resolved all threads

added 1243 commits

• 43e994c9...512d7b63 - 1240 commits from branch master
• b404b456 - Add note about env slugs being immutable
• 7dc4270e - Apply suggestion to doc/ci/environments.md
• 47b86049 - Fix docs linting

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assigned to @eread and unassigned @steveazz and @anton

added grouprelease [DEPRECATED] label

enabled an automatic merge when the pipeline for 47b86049 succeeds

approved this merge request

merged

mentioned in commit 3f9ce7a1

added docsimprovement label

mentioned in issue gitlab-com/support/support-training#1620 (closed)

added typemaintenance label