Add information about XML namespaces

[timunie]

Add docs about this:

• How to add a namespace via xmlns:my1="clr-namespace:My.NameSpace"
• How to add a namespace via xmlns:my2="clr-namespace:My.NameSpace.InOtherAssembly;;assembly=TheOtherAssembly"
• How to add a namespace via xmlns:my3="using:My.NameSpace"
• How to add a namespace via xmlns:myCommonNS:"www.myUrl.com" and how to set up the www.myUrl.com (or similar string) in C#

Question to the team: Where should this docs be added?

1. in here: https://docs.avaloniaui.net/guides/basics/introduction-to-xaml
2. or in here: https://docs.avaloniaui.net/docs/getting-started [New Sub-Page]