Add Cake target to auto-generate API documentation with DocFX

[secana]

Use DocFx to generate the API documentation for the project.

On hold: Currently (30.09.17) DocFx cannot extract an API documentation from an project in .Net Standard 2.0

It does not find the correct MSBuild for the job. There is an issue on Github for it but no real solution.

[secana]

Checked if docfx can generate docu for .net standard. Still not possible. Will check later again.