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/dma/dmamux.rs

@@ -49,5 +49,5 @@ foreach_dma_channel! {
 
 /// safety: must be called only once
 pub(crate) unsafe fn init() {
-    crate::generated::init_dmamux();
+    crate::_generated::init_dmamux();
 }