Update dotnet-core docs
Update dotnet-core docs

- Add SDK selection info
- Buildpack.yml can be used to specify .NET SDK version and version line
- Remove deprecated project.json
- Add precedence list for .NET SDK versioning

[#157975969]

Signed-off-by: Andrew Meyer <8f92422f5861fc9f31873f75dd770facd670512b@pivotal.io>

cloudfoundry/docs-buildpacks