Update usage docs to include creating winappsdk apps

[asklar]

Once we are ready for 1.0 we should update the docs, unhide the CLI flag, etc.

[chrisglein]

Note that this is not all the docs, just the steps so far (does not include ##9602).

This is the first step in docs, so creating the spot in the docs (alongside UWP, not replacing I assume) and having an article on how to create an app, referring to CLI and samples.