Add Appium test guide

Continuation of !304 (closed)

This is a comprehensive tutorial mentioning several techniques actually used in KDE software to start writing Appium tests.

The hard part was trying to find where each thing belongs to (Selenium, Appium etc) for the documentation links so devs can find what they need to start working.

Also: the group section is called Tests and the page weight is 3 because I plan to add Tests and Autotests in the future (and finally eliminate all the redundant test pages on the wikis).

Live view: https://kdocs.rabbitictranslator.com/appium/public/docs/apps/tests/appium/