docs(examples): introduce summary and description to examples (#9968)

Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

examples/libs/libpng/about.md

@@ -0,0 +1,6 @@
+---
+title: "libpng"
+order: 45
+---
+
+Reference PNG decoder implementation. Enable with `LV_USE_LIBPNG` in `lv_conf.h`.

examples/libs/libpng/lv_example_libpng_1.c

@@ -4,7 +4,14 @@
 #if LV_USE_LIBPNG
 
 /**
- * Open a PNG image from a file
+ * @title PNG from array and file with libpng
+ * @brief Show the same PNG decoded from an embedded array and from a file path.
+ *
+ * Two image widgets are created on the active screen. The left one binds
+ * to the `img_png_demo` descriptor declared with `LV_IMAGE_DECLARE`, and
+ * the right one reads `A:lvgl/examples/libs/libpng/png_demo.png` through
+ * the filesystem driver registered under drive letter `A`. Both routes
+ * go through the libpng image decoder.
  */
 void lv_example_libpng_1(void)
 {