Add docs/reference split (exercism#795)

* [Docs] Move implementing-a-concept-exercise.md to reference folder
* [Docs] Move reference documents to docs folder
* [Docs] Update to student-facing docs
* [Docs] Add new issue template

Co-Authored-By: Jeremy Walker <jez.walker@gmail.com>
Co-Authored-By: Victor Goff <keeperotphones@gmail.com>
Co-authored-by: Sascha Mann <git@mail.saschamann.eu>

reference/implementing-a-concept-exercise.md

@@ -0,0 +1,5 @@
+# How to implement an Python concept exercise
+
+TODO: describe how to implement a concept exercise for the Python track. For inspiration, check out the [C# version of this file][csharp-implementing].
+
+[csharp-implementing]: ../../csharp/reference/implementing-a-concept-exercise.md