Add registry limit (5) and upstream limit (5) to the docs

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We have a maximum of 5 virtual registries per group, and 5 upstreams to each virtual registry. We should reflect this in the docs:

• https://docs.gitlab.com/api/container_virtual_registries/#create-a-virtual-registry
• https://docs.gitlab.com/api/container_virtual_registries/#create-an-upstream-registry
• https://docs.gitlab.com/api/container_virtual_registries/#associate-an-upstream-with-a-registry

Feel free to add it to more docs as you see fit.