Introduce C# MaD documentation

Author: egregius313

docs/codeql/codeql-language-guides/codeql-for-csharp.rst

@@ -11,11 +11,13 @@ Experiment and learn how to write effective and efficient queries for CodeQL dat
    basic-query-for-csharp-code
    codeql-library-for-csharp
    analyzing-data-flow-in-csharp
+   customizing-library-models-for-csharp
 
 -  :doc:`Basic query for C# code <basic-query-for-csharp-code>`: Learn to write and run a simple CodeQL query.
 
 -  :doc:`CodeQL library for C# <codeql-library-for-csharp>`: When you're analyzing a C# program, you can make use of the large collection of classes in the CodeQL library for C#.
 
 -  :doc:`Analyzing data flow in C# <analyzing-data-flow-in-csharp>`: You can use CodeQL to track the flow of data through a C# program to its use.
 
+-  :doc:`Customizing library models for C# <customizing-library-models-for-csharp>`: You can model frameworks and libraries that your code base depends on using data extensions and publish them as CodeQL model packs.