Document group_link group attribute for stages.yml

Why is this change being made?

This MR documents the attribute group_link for data/stages.yml. It is used to generate the links in the stage graphs, e.g. Growth section. If it is empty, the link will be broken (i.e. handbook.gitlab.com/handbook/<no%20value>).

See also gitlab-com/www-gitlab-com!134432, which adds missing group_link attributes and prompted the attribute to be documented.

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Commits

• Document group_link group attribute for stages.yml

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