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/.cargo/config.toml

@@ -0,0 +1,11 @@
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+runner = "probe-run --chip STM32L475VG"
+
+rustflags = [
+    "-C", "link-arg=--nmagic",
+    "-C", "link-arg=-Tlink.x",
+    "-C", "link-arg=-Tdefmt.x",
+]
+
+[build]
+target = "thumbv7em-none-eabihf"