Add document to describe the representation process for mentors

[sanderploegsma]

We should add the exercises/shared/.docs/representations.md file to describe to mentors how the representations are generated. This content is displayed when a mentor uses the 'Automation' tab on the website.

Information on what this file should contain can be found here: https://exercism.org/docs/building/tracks/shared-files#h-file-representations-md