.NET 7.0: gRPC JSON Transcoding: Add summary section on annotation

[wadepickett]

Provide a summary on annotation in the gRPC Transcoding topic.

We point to this following reference, but a summary is also needed: https://cloud.google.com/service-infrastructure/docs/service-management/reference/rpc/google.api#google.api.HttpRule

This item is not tied to the What's new 7.0 topic updates, we already mention Transcoding there.

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• ID: dfa3c933-30c4-eb49-d9a5-507aff44ce00
• Version Independent ID: 1485c456-eb24-a446-e296-dd3686456837
• Content: gRPC JSON transcoding in ASP.NET Core gRPC apps
• Content Source: aspnetcore/grpc/httpapi.md
• Product: aspnet-core
• Technology: aspnetcore-grpc
• GitHub Login: @JamesNK
• Microsoft Alias: jamesnk