hub: refresh gated distribution

[craig-osterhout]

Description

Update Gated Distribution docs with clearer conceptual info and runnable API examples

• Added a new Distributor members section explaining the role/access model — team-scoped pull-only access, member/repo isolation, and guidance to provision a separate org to avoid consuming distributor licenses. This folds in content Enablement shared as FAQs, reframed as conceptual explanation rather than a Q&A list.
• Removed the "Key features" bullet list (it read like a marketing callout out of place in a task-oriented access-management doc) and merged the non-redundant points into the intro paragraph.
• Rewrote Invite distributor members via API with copy-pasteable curl examples for every step (generate token, create team, grant repo access, send bulk invites), chained via $TOKEN/$GROUP_ID so the whole flow can be run end-to-end. UI vs. API alternatives are now presented as tabs instead of nested bullets.

Preview: https://deploy-preview-25933--docsdocker.netlify.app/docker-hub/repos/manage/access/#gated-distribution

Related issues or tickets

ENGDOCS-3369

Reviews

• Technical review
• Editorial review
• Product review

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