Feature/#114 issue templates

Summary

This MR provides issue templates (bug, feature request) for GitLab to improve our development workflow.

Use Cases

We can use this for every bug and feature request issue.

Documentation

Are there parts of the documentation we have to adjust?

• Yes, and I already did! (in CONTRIBUTING.md)

Tests

Are we able to test this new feature?

• Not applicable.

Changelog

• I'm not sure if this makes sense since it's not an enhancement to the software itself.

Related Tickets

Closes

Closes #114 (closed).

/cc @mrodzis @mgoebel