Add PingCAP's `TiDB Introduction` book (#6954)

* Update free-programming-books-langs.md

Added a new section NewSQL and added the first course in the section

* add toc entry for `NewSQL`

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

books/free-programming-books-langs.md

@@ -137,6 +137,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Neo4J](#neo4j)
 * [.NET Core / .NET](#net-core)
 * [.NET Framework](#net-framework)
+* [NewSQL](#newsql)
 * [Nim](#nim)
 * [NoSQL](#nosql)
 * [Oberon](#oberon)
@@ -1550,6 +1551,11 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Visual Studio 2019 Succinctly](https://www.syncfusion.com/ebooks/visual-studio-2019-succinctly) - Alessandro Del Sole
 
 
+### NewSQL
+
+* [TiDB Introduction](https://docs.pingcap.com/tidb/stable) - PingCAP [(PDF)](https://download.pingcap.org/tidb-stable-en-manual.pdf)
+
+
 ### Nim
 
 * [Computer Programming with the Nim Programming Language](http://ssalewski.de/nimprogramming.html) - Dr. Stefan Salewski