Add free c# programming online book (#7166)

* Add free c# programming online book

Add C# programming in thai language.

* lint: fix warnings

* register C# in TOC

* fix resource metadata

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-th.md

@@ -1,6 +1,7 @@
 ### Index
 
 * [Apache Spark](#apache-spark)
+* [C#](#csharp)
 * [Go](#go)
 * [IoT (internet of things)](#iot-internet-of-things)
 * [Java](#java)
@@ -12,6 +13,11 @@
 * [Spark Internals](https://github.com/JerryLead/SparkInternals/tree/HEAD/markdown/thai) - Lijie Xu, Bhuridech Sudsee
 
 
+### <a id="csharp"></a>C\#
+
+* [บทเรียนภาษา C#](http://marcuscode.com/lang/csharp) - MarcusCode
+
+
 ### Go
 
 * [ภาษา Go ตอน 1 ติดตั้ง และ Run Hello World](https://medium.com/odds-team/%E0%B8%AA%E0%B8%A3%E0%B8%B8%E0%B8%9B%E0%B8%81%E0%B8%B2%E0%B8%A3%E0%B9%80%E0%B8%A3%E0%B8%B5%E0%B8%A2%E0%B8%99%E0%B8%9E%E0%B8%B7%E0%B9%89%E0%B8%99%E0%B8%90%E0%B8%B2%E0%B8%99%E0%B8%A0%E0%B8%B2%E0%B8%A9%E0%B8%B2-go-%E0%B9%81%E0%B8%9A%E0%B8%9A-step-by-step-%E0%B8%88%E0%B8%B2%E0%B8%81-course-pre-ultimate-go-by-p-yod-%E0%B8%95%E0%B8%AD%E0%B8%99-1-%E0%B8%95%E0%B8%B4%E0%B8%94%E0%B8%95%E0%B8%B1%E0%B9%89%E0%B8%87-%E0%B9%81%E0%B8%A5%E0%B8%B0-d9ac7913e9a4) - Chaiyarin Niamsuwan