Add docs/reference split
Add docs/reference split

* [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 <748be3168291910db6bd9ae2b7d028582a439c01@gmail.com>
Co-Authored-By: Victor Goff <e059ac4c63c41969c1b35067ad605f85695225dc@gmail.com>
Co-authored-by: Sascha Mann <46f1a0bd5592a2f9244ca321b129902a06b53e03@mail.saschamann.eu>

exercism/xocaml