Don't create tool manifests in a .config subfolder by default

[dsplaisted]

A tool manifest file (dotnet-tools.json) can either be directly in the folder it applies to, or it can be in a .config subfolder. The idea behind this was to group multiple configuration files in the .config folder in order to avoid cluttering the repo root.

However, we never ended up adding any other files to the .config subfolder. So now we'd like to simplify local tools by putting the manifest file directly into the folder it applies to.

This will involve updating the tool-manifest template, as well as the code in the .NET CLI that creates the manifest if necessary.

Documentation, such as this page, should also be updated.