Updated _Sidebar (markdown)

_Sidebar.md

@@ -22,6 +22,7 @@
 * [Working on Raspberry Pi](Working-on-Raspberry-Pi)
 * [Working for Android](Working-for-Android)
 * [Working for Web (HTML5)](Working-for-Web-(HTML5))
+* [Creating Discord Activities](Creating-Discord-Activities-with-raylib)
 * [Working anywhere with CMake](Working-with-CMake)
 * [CMake Build Options](CMake-Build-Options)