docs: add proposal issue template

[hifabienne]

Definition of Ready

• [ ] I am happy with the code
• [ ] Short description of the feature/issue is added in the pr description
• [ ] PR is linked to the corresponding user story
• [ ] Acceptance criteria are met
• [ ] All open todos and follow ups are defined in a new ticket and justified
• [ ] Deviations from the acceptance criteria and design are agreed with the PO and documented.
• [ ] No debug or dead code
• [ ] My code has no repetitions
• [ ] Critical parts are tested automatically
• [ ] Where possible E2E tests are implemented
• [ ] Documentation/examples are up-to-date
• [ ] All non-functional requirements are met
• [ ] Functionality of the acceptance criteria is checked manually on the dev system.