Add minimal no_std example.

src/SUMMARY.md

@@ -237,6 +237,7 @@
 
 - [Welcome](welcome-bare-metal.md)
 - [`no_std`](bare-metal/no_std.md)
+  - [A minimal example](bare-metal/minimal.md)
   - [`core`]()
   - [`alloc`]()
 - [Microcontrollers]()

src/bare-metal/minimal.md

@@ -0,0 +1,22 @@
+# A minimal `no_std` program
+
+```rust,editable
+#![no_main]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[panic_handler]
+fn panic(_panic: &PanicInfo<'_>) -> ! {
+    loop {}
+}
+```
+
+<details>
+
+* This will compile to an empty binary.
+* `std` provides a panic handler; without it we must provide our own.
+* Depending on the target, you may need to compile with `panic = "abort"` to avoid an error about
+  `eh_personality`.
+
+</details>