Add .NET Quickstart

[berviantoleo]

I would like to add .NET quickstart. However, I'll add it step by step.

Here is my plan.

1. Preparing the agent (in this repository)
2. Preparing the example (I'll prepare myself, but may need jenkins-docs org to host it later)
3. Update this repo to pull the example, for testing purposes.
4. Add a new tutorial to jenkins.io

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• Agent: feat: add dotnet agent #1057
• Sample App: https://github.com/berviantoleo/simple-dotnet-web-app

[gounthar]

Thanks, Bervianto.