Complete toolkit documentation

[jamesmcroft]

There is a new docs folder in the project that will contain all of the documentation for the toolkit and it will be hosted here: https://jamesmcroft.github.io/WinUX-UWP-Toolkit/

Under the API folder, there are currently two folders called namespace and file. All of the root pages for namespaces are complete but there is a lot of work to do to update all of the file pages. 

If anyone could spare anytime to do one or two, that would be great. I'll update soon on the format preferred for writing them but you can check out the examples that already exist in the folders.