Add nuget.config to repo root

From conversation with @heaths: Move NuGet.config.template to nuget.config at the root of the repo.

It's better to have this defined explicitly than rely on processes that differ between dev boxes and compliant build pipelines.

Start by inspecting NuGet.config.template and the repository root. Move the template to nuget.config at the root, then verify that the explicitly named configuration is present and no longer depends on differing developer or build-pipeline processes.

Written by the indexing model from the issue text.