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/assets.h

@@ -1,8 +1,20 @@
+/**
+ * @file assets.h
+ * @brief Declares the public assets API.
+ */
+
 #ifndef _ASSETS_H_
 #define _ASSETS_H_
 
 #include <akerror.h>
 
+/**
+ * @brief Load start bgm.
+ * @param fname Path to the input or output file.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When the corresponding validation or operation fails.
+ * @throws AKGL_ERR_SDL When the corresponding validation or operation fails.
+ */
 akerr_ErrorContext AKERR_NOIGNORE *akgl_load_start_bgm(char *fname);
 
 #endif //_ASSETS_H_