Update doc/api/groups.md

What does this MR do?

Currently the documentation regarding how to use the API to move projects is lacking clarity and an example on how to perform the API request via curl. Specifically, it is unclear if the POST request needs any data supplied nor does it specify if the group ID should be for the target group or source group.

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

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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

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2. Ensure a release milestone is set.
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