Update the C# client page to use newer version of the client library (#13784)
Update the C# client page to use newer version of the client library (#13784)

Motivation
The current version of the docs uses version 0.11 of the C# client library, and the newest version of that library is now 2.0.1.
We need to update the c# client doc to match the new version.

Modification
* Update the c# client doc
* Add import codes where necessary

Signed-off-by: Zike Yang <e13d820f4a041c136f3334569af48fd3c9f6f772@streamnative.io>
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