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_platform.h

@@ -20,9 +20,10 @@
 */
 
 /**
- *  \file SDL_platform.h
+ * # CategoryPlatform
  *
- *  Header file for platform functions.
+ * SDL provides a means to identify the app's platform, both at compile time
+ * and runtime.
  */
 
 #ifndef SDL_platform_h_