boards: google: adopt new zephyr:board directive and role

This updates the documentation of all the Google boards to use
the new `zephyr:board::` directive.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

boards/google/dragonclaw/doc/index.rst

@@ -1,7 +1,4 @@
-.. _google_dragonclaw_board:
-
-Google Dragonclaw Development Board
-###################################
+.. zephyr:board:: google_dragonclaw
 
 Overview
 ********

boards/google/twinkie_v2/doc/index.rst

@@ -1,7 +1,4 @@
-.. _google_twinkie_v2_board:
-
-Google Twinkie V2
-#################
+.. zephyr:board:: google_twinkie_v2
 
 Overview
 ********

samples/boards/google/twinkie_v2/pda/README.rst

@@ -7,7 +7,7 @@
 Overview
 ********
 
-This provides access to :ref:`Twinkie <google_twinkie_v2_board>` so you can try out
+This provides access to :zephyr:board:`google_twinkie_v2` so you can try out
 the supported features.
 
 Building and Running