Add .NET ton_proof demo (ton-community#723)

Author: WinoGarcia

docs/develop/dapps/ton-connect/sign.mdx

@@ -212,6 +212,7 @@ The signature must be verified by public key:
 * [JS demo app](https://github.com/liketurbo/demo-dapp-backend-js)
 * [Python example](https://github.com/XaBbl4/pytonconnect/blob/main/examples/check_proof.py)
 * [PHP example](https://github.com/vladimirfokingithub/Ton-Connect-Proof-Php-Check)
+* [C# demo app](https://github.com/WinoGarcia/TonProof.NET)
 
 ## See Also