stm32: exclude spammy files from docs by prefixing with _.

These files end up containing chip-specific info, so they deduplicate
really badly when building docs for all 1200 chips.

The doc generator removes files starting with `_`.

embassy-stm32/src/gpio.rs

@@ -574,7 +574,7 @@ foreach_pin!(
 );
 
 pub(crate) unsafe fn init() {
-    crate::generated::init_gpio();
+    crate::_generated::init_gpio();
 }
 
 mod eh02 {