Docs: Supporting a keyboard in the QMK Configurator (#4396)

Docs: Supporting a keyboard in the QMK Configurator (#4396)

* Docs: Configurator Support

* Minor updates

* Updated keyboard.h example to use #pragma once

per @drashna
https://github.com/qmk/qmk_firmware/pull/4396#discussion_r232140934

* Add link to doc under Reference section

Added to _sidebar.md and _summary.md

* Changed doc title; minor tweaks

Re-titled the doc as "Supporting Your Keyboard in QMK Configurator".
Made some minor changes to the keyboard.h examples.

* Added tags object to JSON example