Add RepositoryOverview.md

This file is a high level description of the current structure of the
repository.
It contains some information for new developers,
which might be useful for them to get acquainted with the system.

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I wasn't sure where to put the image, so I kinda followed the current `docs` folder convention.
As the document is aimed at new contributors, I just put it in the main repo, but I can understand if you'd rather have me put it in AsciiDoc format and add it to the contributing section of the manual.

I hope this document can be of use.
Feel free to tell me what to change if this does not meet your standards.