Add documentation about the different embassy abstraction layers

The guide demonstrates the functionality offered by each
layer in Embassy, using code examples.

docs/modules/ROOT/examples/layer-by-layer/blinky-hal/src/main.rs

@@ -0,0 +1,23 @@
+#![no_std]
+#![no_main]
+
+use cortex_m_rt::entry;
+use defmt_rtt as _;
+use panic_probe as _;
+
+use embassy_stm32::gpio::{Input, Level, Output, Pull, Speed};
+
+#[entry]
+fn main() -> ! {
+    let p = embassy_stm32::init(Default::default());
+    let mut led = Output::new(p.PB14, Level::High, Speed::VeryHigh);
+    let button = Input::new(p.PC13, Pull::Up);
+
+    loop {
+        if button.is_low() {
+            led.set_high();
+        } else {
+            led.set_low();
+        }
+    }
+}