exercism-matrix: add introduction

[ErikSchierboom]

Add contents to the introduction.md document of the exercise, which teaches one concept: matrices-introduction
This should either be renamed to matrices, or split up into several matrices-X concepts.

The corresponding concept documents should also be written

[rainerrodrigues]

Is this issue closed or can I take it up? @colinleach @ErikSchierboom

[colinleach]

Not for me to say - the maintainer is @cmcaine.

[ErikSchierboom]

I'd be happy for you to work on this, but let's hear what @cmcaine has to say.

[cmcaine]

Thanks for your interest, rainerrodrigues. I would be happy to review and merge a PR that addresses this issue.

[colinleach]

See #903.