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

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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#### Document Details

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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](https://docs.microsoft.com/en-us/aspnet/core/grpc/httpapi?view=aspnetcore-7.0)
* Content Source: [aspnetcore/grpc/httpapi.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/grpc/httpapi.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-grpc**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**

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[Associated WorkItem - 33233](https://dev.azure.com/msft-skilling/Content/_workitems/edit/33233)