wikiheaders: bridge wiki Category docs to the headers!

Did an initial cleanup on the headers and wrote a few pieces of documentation,
but this needs more work to fill out the documentation.

include/SDL3/SDL_system.h

@@ -20,9 +20,9 @@
 */
 
 /**
- *  \file SDL_system.h
+ * # CategorySystem
  *
- *  Include file for platform specific SDL API functions
+ * Platform-specific SDL API functions.
  */
 
 #ifndef SDL_system_h_