Implements Docfx for aspnetboilerplate api docs

[demirmusa]

Run create-docs.ps1 file to create docs.

You can add -s parameter to serve after creation completed (.\create-docs.ps1 -s). Or you can run serve.ps1 after creation completed

Note: You must have Docfx to able to create docs. https://dotnet.github.io/docfx/tutorial/docfx_getting_started.html

closes #6439