Guide: Clarify that Level can be used as an output signal (#2176)

* Clarify that Level can be used as an output signal

* Implement Peripheral for Level

esp-hal/MIGRATING-0.20.md

@@ -172,7 +172,7 @@ configure an input pin, and pass it to `set_edge_signal` or `set_ctrl_signal`.
 Use `NoPin` in place of the now-removed `NO_PIN` constant.
 
 SPI pins, when using the `with_pin` function, are no longer optional.
-You can pass `NoPin` or `Level` as inputs, and `NoPin` as output if you don't need a particular pin.
+You can pass `NoPin` or `Level` if you don't need a particular pin.
 
 ```diff
  let spi = Spi::new(peripherals.SPI2, 100.kHz(), SpiMode::Mode0)

esp-hal/src/gpio/placeholder.rs

@@ -7,6 +7,14 @@
 
 use super::*;
 
+impl crate::peripheral::Peripheral for Level {
+    type P = Self;
+
+    unsafe fn clone_unchecked(&mut self) -> Self::P {
+        *self
+    }
+}
+
 impl PeripheralSignal for Level {
     fn pull_direction(&self, _pull: Pull, _internal: private::Internal) {}
 }