Add Maven Virtual Registry API documentation with endpoints and examples

What does this MR do and why?

Adds documentation for the following Maven virtual registry restful API endpoints:

• API::VirtualRegistries::Packages::Maven::Registries
• API::VirtualRegistries::Packages::Maven::Upstreams
• API::VirtualRegistries::Packages::Maven::Endpoints
• API::VirtualRegistries::Packages::Maven::Cache::Entries

References

• Maven Virtual Registry: Documentation (#468115 - closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

MR acceptance checklist

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Related to #468115 (closed)