Create .NET 10 RC 1 "What's New" topic.

### Describe the issue or suggestion

Create the update for .NET 10 RC 1 using the instructions found here: https://github.com/dotnet/docs/blob/main/.github/prompts/whats-new-net.md

This is a release candidate preview release, so the folder named as "preview<n>" in the prompt is "rc1".

