chore: use forms for opening GitHub issues

[gruyaume]

Description

We use forms for opening issue templates, making it more straightforward for users to fill the appropriate content in the right place and to enforce mandatory fields. We also add a "enhancement" form.

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules