doc: stm32g0: drop redundant @ingroup and close groups

the group defaults to the implicit container based on location, so drop
all the explicit @ingroups, less to maintain.  Properly use /**@}*/ to
close all groups too, even though it mostly seems to have worked anyway.
Properly close all groups opened for files.

lib/stm32/g0/flash.c

@@ -81,3 +81,4 @@ void flash_lock(void)
 	flash_lock_progmem();
 }
 
+/**@}*/