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-async/src/main.rs

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+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt_rtt as _;
+use panic_probe as _;
+
+use embassy::executor::Spawner;
+use embassy_stm32::{
+    exti::ExtiInput,
+    gpio::{Input, Level, Output, Pull, Speed},
+    Peripherals,
+};
+
+#[embassy::main]
+async fn main(_s: Spawner, p: Peripherals) {
+    let mut led = Output::new(p.PB14, Level::Low, Speed::VeryHigh);
+    let mut button = ExtiInput::new(Input::new(p.PC13, Pull::Up), p.EXTI13);
+
+    loop {
+        button.wait_for_any_edge().await;
+        if button.is_low() {
+            led.set_high();
+        } else {
+            led.set_low();
+        }
+    }
+}