docs: document assigning collections to resource groups

[Kakulukian]

Collections can now be assigned to a Resource Group (Team & Enterprise), following the same access-control and visibility rules as repositories.

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Note

Low Risk
Documentation-only updates with no application, API, or security logic changes.

Overview
Documents Team & Enterprise support for tying organization collections to Resource Groups, mirroring repository access control.

collections.md adds a section on choosing a Resource Group at creation, moving collections via the collection menu, and limiting visibility of private collections to group members, with a link to security-resource-groups.

security-resource-groups.md states that collections follow the same one-group-per-resource rules as repos and that private collections in a group use the same visibility rules as private repos in that group.

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[HuggingFaceDocBuilderDev]

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