Rename de_feature_request to de_feature_request.md (#206)

**IMPORTANT: Please do not create a Pull Request without creating an
issue first.**

## Summary

**Issue:** #204 

Add file extension `.md`.

## Type (Fill in "x" to check)

- [x] Bug Fix
- [ ] New Feature

## Checklist

Please make sure that all items are checked before submitting this
request.

- [x] Code linter has been run and issues have all been resolved
- [x] The code has been thoroughly tested and no visible bugs have been
introduced
- [x] The pull request will completely resolve the issue(s) mentioned
- [x] The pull request only resolves the issue(s) mentioned and nothing
more

## Additional context

N/A