boards: riscv: longan_nano: Update document

Improve documentation

- Add a photo of the board
- Add hardware information from the board manual
- Fix options for build (set OPENOCD_DEFAULT_PATH to make it
  valid for also windows environment)
- Improve the build instructions to make it easier to understand

Signed-off-by: Yuichiro SAGISAKA <yu.sagisaka@fujitsu.com>

Author: sagisaka0328

boards/riscv/longan_nano/doc/img/longan_nano.png

@@ -3,6 +3,10 @@
 Sipeed Longan Nano
 ##################
 
+.. image:: img/longan_nano.png
+     :align: center
+     :alt: longan_nano
+
 Overview
 ********
 
@@ -17,21 +21,68 @@ More information can be found on:
 - `Nuclei Bumblebee core documents <https://github.com/nucleisys/Bumblebee_Core_Doc>`_
 - `Nuclei ISA Spec <https://doc.nucleisys.com/nuclei_spec/>`_
 
-Programming and debugging
-*************************
-
-Building
-========
+Hardware
+********
 
-Applications for the ``logan_nano`` board configuration can be built as usual
-(see :ref:`build_an_application`) using the corresponding board name:
+- 4 x universal 16-bit timer
+- 2 x basic 16-bit timer
+- 1 x advanced 16-bit timer
+- Watchdog timer
+- RTC
+- Systick
+- 3 x USART
+- 2 x I2C
+- 3 x SPI
+- 2 x I2S
+- 2 x CAN
+- 1 x USBFS(OTG)
+- 2 x ADC(10 channel)
+- 2 x DAC
+
+Supported Features
+==================
+
+The board configuration supports the following hardware features:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Peripheral
+     - Kconfig option
+     - Devicetree compatible
+   * - GPIO
+     - :kconfig:option:`CONFIG_GPIO`
+     - :dtcompatible:`gd,gd32-gpio`
+   * - Machine timer
+     - :kconfig:option:`CONFIG_RISCV_MACHINE_TIMER`
+     - :dtcompatible:`riscv,machine-timer`
+   * - Nuclei ECLIC Interrupt Controller
+     - :kconfig:option:`CONFIG_NUCLEI_ECLIC`
+     - :dtcompatible:`nuclei,eclic`
+   * - PWM
+     - :kconfig:option:`CONFIG_PWM`
+     - :dtcompatible:`gd,gd32-pwm`
+   * - USART
+     - :kconfig:option:`CONFIG_SERIAL`
+     - :dtcompatible:`gd,gd32-usart`
+   * - I2C
+     - :kconfig:option:`CONFIG_I2C`
+     - :dtcompatible:`gd,gd32-i2c`
+   * - DAC
+     - :kconfig:option:`CONFIG_DAC`
+     - :dtcompatible:`gd,gd32-dac`
+
+Serial Port
+===========
+
+USART0 is on the opposite end of the USB.
+Connect to TX0 (PA9) and RX0 (PA10).
 
-.. zephyr-app-commands::
-   :board: logan_nano
-   :goals: build
+Programming and debugging
+*************************
 
-Flashing
-========
+Building & Flashing
+===================
 
 In order to upload the application to the device, you'll need OpenOCD with
 GD32V support. Download the tarball for your OS from the
@@ -40,25 +91,28 @@ GD32V support. Download the tarball for your OS from the
 
 The Zephyr SDK uses a bundled version of OpenOCD by default. You can
 overwrite that behavior by adding the
-``-DOPENOCD=<path/to/riscv-openocd/bin/openocd>``
+``-DOPENOCD=<path/to/riscv-openocd/bin/openocd> -DOPENOCD_DEFAULT_PATH=<path/to/riscv-openocd/share/openocd/scripts>``
 parameter when building:
 
+Here is an example for building the :ref:`blinky-sample` application.
+
 .. zephyr-app-commands::
+   :zephyr-app: samples/basic/blinky
    :board: longan_nano
-   :goals: build
-   :gen-args: -DOPENOCD=<path/to/riscv-openocd/bin/openocd>
+   :goals: build flash
+   :gen-args: -DOPENOCD=<path/to/riscv-openocd/bin/openocd> -DOPENOCD_DEFAULT_PATH=<path/to/riscv-openocd/share/openocd/scripts>
 
 When using a custom toolchain it should be enough to have the downloaded
 version of the binary in your ``PATH``.
 
-Now you can flash the application as usual (see :ref:`build_an_application` and
-:ref:`application_run` for more details):
-
-.. code-block:: console
-
-   west flash
-
 Debugging
 =========
 
-Refer to the detailed overview about :ref:`application_debugging`.
+You can debug an application in the usual way.  Here is an example for the
+:ref:`blinky-sample` application.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/basic/blinky
+   :board: longan_nano
+   :maybe-skip-config:
+   :goals: debug