Getting started guide for the Xamarin IDM SDK

Why

See parent epic

What

We have getting started guide for native Android and iOS IDM SDKs.

We should have similar guide for the Xamarin IDM SDK.

It should provide information on:

• Prerequisites (setup keycloak)
• how to add the Xamarin IDM SDK dependency to a project
• how to init the Xamarin IDM SDK
• code example of using the APIs of the Xamarin IDM SDK.

Given that Xamarin is a cross-platform, we might need to provide code samples to cover both iOS and Android platforms.

As part of this work, we also want to:

• Convert the existing markdown files in the `Documentations` directory into asciidocs.
• To follow the same convention as other SDKs, let's move the content from `Documentations` dir to the `docs` dir. See https://github.com/aerogear/aerogear-xamarin-sdk/pull/37