Simplify include repo configuration in ml-api-docs repo

The current config in https://github.com/dotnet/ml-api-docs is:

.openpublishing.publish.config.json

"dependent_repositories": [
...
{
"path_to_root": "docs/samples",
"url": "https://github.com/dotnet/machinelearning",
"branch": "master",
"branch_mapping": {}
}
],

This means that the includes have to be specified like this:

[!code-csharp[OrdinaryLeastSquares](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/Regression/OrdinaryLeastSquares.cs)]

and

[!include[algorithm](~/../docs/samples/docs/api-reference/tree-featurization-prediction.md)]

If we change it to

.openpublishing.publish.config.json

{
"path_to_root": "dotnet/machinelearning", <-- ~ is relative to the docfx root dir i.e. dotnet
"url": "https://github.com/dotnet/machinelearning",
"branch": "master",
"branch_mapping": {}
}

then we can specify the includes as:

[!code-csharp[OrdinaryLeastSquares](~/machinelearning/docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/Regression/OrdinaryLeastSquares.cs)]

[!include[algorithm](~/machinelearning/docs/api-reference/tree-featurization-prediction.md)]