Update external users permissions docs

What does this MR do?

Previously it was unclear what external users' resource creation permissions were meant to be, leading to some personal confusion and security reports. The intended behavior is that external users should be able to create projects/subgroups/snippets within groups they belong to, essentially "confining" them to a single top-level group. They should not be able to create their own top-level groups/projects, and should not be able to create snippets in public projects they do not belong to.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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