docs: Explain set-upstream for stacks in forks

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Document set-upstream behavior for stacks when working with forks

This merge request addresses the need to document and clarify the set-upstream behavior for stacks, particularly when users are working with forks. The changes include:

1. Updated the long description of the glab stack sync command to include information about fork behavior.
2. Utilized create.ResolvedHeadRepo(f)() from the CLI's stack sync command to prompt users for the head repo when working with forks, improving merge request handling in forked environments.

These changes aim to improve user experience by:

• Providing clear documentation about the behavior when working with forks.
• Giving users the option to choose between their fork and the original repository as the head repo for merge requests.
• Ensuring that merge requests are created against the correct repository.

This addresses the discussion from MR !1687 (merged), where it was suggested to document or fix this behavior to prevent unexpected results for users working with forks.

Closes #7660 (closed)