Add dotnet CI/CD example

We don't have any CI/CD examples for .NET, we should add some.

• https://gitlab.com/forrestab/dotnet-gitlab-ci
• http://johnspaetzel.com/gitlabci-dotnet
• https://dotnetthoughts.net/building-dotnet-with-gitlab-ci/
• https://forum.gitlab.com/t/build-net-projects/1395
• https://stackoverflow.com/questions/42354727/using-gitlab-ci-to-build-net-core-projects-in-docker
• https://medium.com/@gabriel.faraday.barros/gitlab-ci-cd-with-net-framework-39220808b18f