Add a new Ada section (EbookFoundation#3228)

* Add a new Ada section

* Update free-programming-interactive-tutorials-en.md

Author: Momozor

free-programming-interactive-tutorials-en.md

@@ -1,5 +1,6 @@
 ### Index
 
+* [Ada](#ada)
 * [Bash](#bash)
 * [C](#c)
 * [C#](#c-sharp)
@@ -31,6 +32,11 @@
 * [Vim](#vim)
 
 
+### Ada
+
+* [Introduction to Ada](https://learn.adacore.com/courses/intro-to-ada/index.html)
+
+
 ### Bash
 
 * [Help messages will explain everything](https://explainshell.com)