Adds troubleshooting for ambiguous HEAD branch

What does this MR do?

This merge request is a child of !105843 (merged). It exists to craft troubleshooting docs for the "ambiguous HEAD" problem described in Update to default branch on project via GUI or ... (#381731 - closed) without the long pipeline in the parent MR.

Important notes:

• Needs a TW review.
• !105843 (merged) needs a separate update after we settle on error message text here.

Other possible text versions

When I created the MR, I was basing it off this wording, but I'm not convinced it's 100% right:

Error: Ambiguous HEAD branch exists. [How do I fix it?]

Other possibilities:

• Error: Can't update the default branch, because a branch named HEAD also exists. [How do I fix it?]
• Error: Failed to update the default branch. A branch named HEAD exists in this repository. [How do I fix it safely?]
• (I can generate more.)

Related issues

• Related to Update to default branch on project via GUI or ... (#381731 - closed)
• Related to !105843 (merged) - this docs work is intended to merge into it

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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Reviewer's checklist

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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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