Add documentation on representations

[sanderploegsma]

The exercises/shared/.docs/representations.md file contents are displayed in the Automations part of the website used by mentors to set up automated mentor feedback on solution sets grouped by their representations. This is explained here: https://exercism.org/docs/building/tracks/shared-files#h-file-representations-md

Fixes #2699

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Reviewer Resources:

Track Policies

[sanderploegsma]

I also noticed that the Elixir track contains more documentation on how these normalizations work: https://github.com/exercism/elixir/blob/d0f255138e0b13eb81e364e2ab6c3a9aa498e540/exercises/shared/.docs/representations.md

I'd like to add this as well, but since the representer might be overhauled soon after merging exercism/java-representer#109 I'd like to hold off on that for now.

[manumafe98]

I'd like to add this as well, but since the representer might be overhauled soon after merging exercism/java-representer#109 I'd like to hold off on that for now.

Do you think we should go ahead and merge this representation until the change is merged or we should wait?

[sanderploegsma]

Currently this file describes the representer as it is running right now, so I'd say we merge this.