added doc on 2-d arrays

[Kjp04]

What is the change?

I have created a doc file on 2-d arrays in c++

Related issue?

closes: #92

Checklist:

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• [x] I have followed the Code of Conduct.
• [x] I have checked there aren't other open Pull Requests for the same update/change.
• [x] I have you tested the code before submission.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generates no new warnings.
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