Documentation Repo: Sphinx warning fixes issue #36

* Fixed the following issues for sphinx warning:
 * about-openharmony and restructuredText Basics topic modified to include figure caption as a single line.
 * quickstart/readme topic modified to include :ref syntax for Supported Boards
 * quickstart/ Linux and Zephyr flavour topic modified for reference
   links in Supported images
 * Coding Guidelines removed from Contribution Process.
 * Avenger, Nitrogen 96 board and seco-imx8mm: .. _linux-image-1: syntax removed for
   duplication
 * Supported boards: Readme file modified to 'include ..
   _supported-boards:' syntax
 *
* Signed off By: Arul Kumaran <arulkumaran.devarajan@huawei.com>