Update docs for testing a new shield.

Clarifying the need for `west flash` in a dockerized environment and providing a high-level overview of testing a board with UF2 support.

docs/docs/development/new-shield.md

@@ -463,12 +463,21 @@ you should be able to test with a build command like:
 west build --pristine -b proton_c -- -DSHIELD=my_board
 ```
 
-and then flash with:
+The above build command generates `build/zephyr/zmk.uf2`.  If your board
+supports USB Flashing Format (UF2), copy that file onto the root of the USB mass
+storage device for your board.  The controller should flash your built firmware
+and automatically restart once flashing is complete.
+
+Alternatively, if your board supports flashing and you're not developing from
+within a Dockerized environment, then you can test your build with:  
 
 ```
 west flash
 ```
 
+Please have a look at documentation specific to
+[building and flashing](build-flash) for additional information.
+
 :::note
 Further testing your keyboard shield without altering the root keymap file can be done with the use of `-DZMK_CONFIG` in your `west build` command,
 shown [here](build-flash#building-from-zmk-config-folder)