Add HCI Zephyr vendor commands to read and write TX power

Create platforms/zephyr/hci.py with definitions of vendor HCI commands
to read and write TX power.

Add documentation for how to prepare an nRF52840 dongle with a Zephyr
HCI USB firmware application that includes dynamic TX power support and
how to send a write TX power vendor HCI command from Bumble.

docs/mkdocs/mkdocs.yml

@@ -64,6 +64,7 @@ nav:
     - Linux: platforms/linux.md
     - Windows: platforms/windows.md
     - Android: platforms/android.md
+    - Zephyr: platforms/zephyr.md
   - Examples:
     - Overview: examples/index.md