Enrich the Xamarin SDK guide in docs

[fungc-io]

The instruction in the quickstart guide is not enough.
A developer should be able to set up a simple login/logout flow and send authenticated requests by ONLY following the quick start guide without looking at the SDK reference.

Please refer to the sections in https://docs.authgear.com/get-started/website

In Get Started:

• Add how to Get the SessionState and UserInfo (ref: https://docs.authgear.com/get-started/website#get-the-logged-in-state)
• Add How to send an authenticated requests to API (ref: https://docs.authgear.com/get-started/website#calling-an-api)

Add Xamarin code samples to other pages:

• https://docs.authgear.com/strategies/biometric
• https://docs.authgear.com/integrate/auth-ui
• https://docs.authgear.com/integrate/reauthentication
• https://docs.authgear.com/integrate/single-sign-on
• https://docs.authgear.com/integrate/force-authentication-on-app-launch