add uarte draft

embedded-workshop-book/src/uarte-implementation.md

@@ -0,0 +1,10 @@
+# Write the Uarte implementation
+## Step-by-Step Solution
+* add field to the board struct
+* add struct for the instance, how to figure out what the type of the inner field is
+* create instance in init, add baudrate, parity etc. 
+* add to instantiation of board struct
+* impl fmt::Write for the Uarte struct, simple write does not work because of dma 
+* example code with button is not a good idea for the simple button implementation.
+
+I think this is plenty for an hour.