Document the libakgl API with Doxygen

Add file, structure, global, and function documentation across all libakgl-owned headers and sources, including parameter contracts and likely AKERR/AKGL_ERR exceptions. Add a strict Doxyfile and build the generated API documentation in Gitea CI with warnings treated as failures.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>

include/akgl/draw.h

@@ -1,6 +1,16 @@
+/**
+ * @file draw.h
+ * @brief Declares the public draw API.
+ */
+
 #ifndef _DRAW_H_
 #define _DRAW_H_
 
+/**
+ * @brief Draw background.
+ * @param w Destination width.
+ * @param h Destination height.
+ */
 void akgl_draw_background(int w, int h);
 
 #endif //_DRAW_H_