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/Cargo.toml

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+[package]
+name = "blinky-hal"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+cortex-m = "0.7"
+cortex-m-rt = "0.7"
+embassy-stm32 = { version = "0.1.0", features = ["stm32l475vg", "memory-x"], default-features = false  }
+
+defmt = "0.3.0"
+defmt-rtt = "0.3.0"
+panic-probe = { version = "0.3.0", features = ["print-defmt"] }

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

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+#![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();
+        }
+    }
+}