Add an introduction that explains the basic format of a Robot Framework Test Suite

[adrianyorke]

Currently, Cookbook dives straight in with recipes showing how to use keywords from certain libraries but there is no explanation of the basic structure and tables that together form a test suite. Beginners especially might not have enough context to understand even the most basic recipe.

There was also a request on Slack to show what you can do with Metadata in the *** Settings *** section.

We should be careful not to repeat the information contained in the User Guide which covers this in some detail.

Please note, this should not be added to beginning of the Table of Contents section as it will renumber all the proceeding sections and require significant refactoring. Perhaps a new section Introduction before Table of Contents.