boards: thingy53_nrf5340: Add brief doc

Add a short summary about the board.

Signed-off-by: Pawel Dunaj <pawel.dunaj@nordicsemi.no>

boards/arm/thingy53_nrf5340/doc/index.rst

@@ -0,0 +1,52 @@
+.. _thingy53_nrf5340:
+
+Thingy:53
+#########
+
+Overview
+********
+
+Zephyr uses the thingy53_nrf5340 board configuration for building
+for the Thingy:53 board. The board has the nRF5340 MCU processor, a set of
+environmental sensors, a pushbutton, and RGB LED.
+
+The nRF5340 is a dual-core SoC based on the Arm® Cortex®-M33 architecture, with:
+
+* a full-featured Arm Cortex-M33F core with DSP instructions, FPU, and
+  Armv8-M Security Extension, running at up to 128 MHz, referred to as
+  the **application core**
+* a secondary Arm Cortex-M33 core, with a reduced feature set, running at
+  a fixed 64 MHz, referred to as the **network core**.
+
+The nrf5340dk_nrf5340_cpuapp build target provides support for the application
+core on the nRF5340 SoC. The nrf5340dk_nrf5340_cpunet build target provides
+support for the network core on the nRF5340 SoC.
+
+
+The `Nordic Semiconductor Infocenter`_ contains the processor's information and
+the datasheet.
+
+Programming and Debugging
+*************************
+
+Flashing
+========
+
+Flashing Zephyr onto Thingy:53 requires an external J-Link programmer. The
+programmer is attached to the P9 programming header.
+
+Debugging
+=========
+
+Thingy:53 does not have an on-board J-Link debug IC as some other nRF5
+development boards, however, instructions from the :ref:`nordic_segger` page
+also apply to this board, with the additional step of connecting an external
+debugger. A development board with a Debug out connector such as the
+:ref:`nrf5340dk_nrf5340` can be used as a debugger with Thingy:53.
+
+References
+**********
+
+.. target-notes::
+
+.. _Nordic Semiconductor Infocenter: http://infocenter.nordicsemi.com/