Update documentation/variants.md: extend tree view

CMakeLists and variants.h should be shown in the tree view.

Co-authored-by: Mike Szczys <[email protected]>

Author: DhruvaG2000

documentation/variants.md

@@ -10,8 +10,11 @@
 ```tree
 variants/
 ├── arduino_nano_33_ble
-   ├── arduino_nano_33_ble.overlay
-   └── arduino_nano_33_ble_pinmap.h
+│   ├── arduino_nano_33_ble.overlay
+│   └── arduino_nano_33_ble_pinmap.h
+├── CMakeLists.txt
+└── variants.h
+
 ```
 
 - The top level consists of `CMakeLists.txt`, `variants.h` and the `<BOARD_NAME>` folder. Each of these files have a specific role to play.