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-pac/Cargo.toml

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+[package]
+name = "blinky-pac"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+cortex-m = "0.7"
+cortex-m-rt = "0.7"
+stm32-metapac = { version = "0.1.0", features = ["stm32l475vg", "memory-x"] }
+
+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-pac/src/main.rs

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+#![no_std]
+#![no_main]
+
+use defmt_rtt as _;
+use panic_probe as _;
+
+use stm32_metapac as pac;
+
+use pac::gpio::vals;
+
+#[cortex_m_rt::entry]
+fn main() -> ! {
+    // Enable GPIO clock
+    let rcc = pac::RCC;
+    unsafe {
+        rcc.ahb2enr().modify(|w| {
+            w.set_gpioben(true);
+            w.set_gpiocen(true);
+        });
+
+        rcc.ahb2rstr().modify(|w| {
+            w.set_gpiobrst(true);
+            w.set_gpiocrst(true);
+            w.set_gpiobrst(false);
+            w.set_gpiocrst(false);
+        });
+    }
+
+    // Setup button
+    let gpioc = pac::GPIOC;
+    const BUTTON_PIN: usize = 13;
+    unsafe {
+        gpioc
+            .pupdr()
+            .modify(|w| w.set_pupdr(BUTTON_PIN, vals::Pupdr::PULLUP));
+        gpioc
+            .otyper()
+            .modify(|w| w.set_ot(BUTTON_PIN, vals::Ot::PUSHPULL));
+        gpioc
+            .moder()
+            .modify(|w| w.set_moder(BUTTON_PIN, vals::Moder::INPUT));
+    }
+
+    // Setup LED
+    let gpiob = pac::GPIOB;
+    const LED_PIN: usize = 14;
+    unsafe {
+        gpiob
+            .pupdr()
+            .modify(|w| w.set_pupdr(LED_PIN, vals::Pupdr::FLOATING));
+        gpiob
+            .otyper()
+            .modify(|w| w.set_ot(LED_PIN, vals::Ot::PUSHPULL));
+        gpiob
+            .moder()
+            .modify(|w| w.set_moder(LED_PIN, vals::Moder::OUTPUT));
+    }
+
+    // Main loop
+    loop {
+        unsafe {
+            if gpioc.idr().read().idr(BUTTON_PIN) == vals::Idr::LOW {
+                gpiob.bsrr().write(|w| w.set_bs(LED_PIN, true));
+            } else {
+                gpiob.bsrr().write(|w| w.set_br(LED_PIN, true));
+            }
+        }
+    }
+}