feat: update ticket templates (add 'definition of done' and user stories)

Summary

This MR provides an update to all ticket and merge request templates. It adds paragraphs for user stories and the 'definition of done'.

Use Cases

Useful for everyone writing tickets.

Tests

Are we able to test this new feature?

• Yes, you can test by following these steps:
  1. open a new issue
  2. select a template beneath the 'Description' label.

Same holds for merge requests.

Changelog

• A CHANGELOG is only available after the other MR have been merged..

Related Tickets

Closes

#15 (closed)

/cc Mathias Göbel, Frank Schneider, Michelle Weidling