github: Add feature request issue template

Author: stephane-caron

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,17 @@
+---
+name: 💡 Feature request
+about: Suggest an idea to improve Upkie
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+⚠️ **Please open a discussion in the [Ideas](https://github.com/robot-descriptions/robot_descriptions.py/discussions/categories/ideas) forum rather than an issue.**
+
+Make sure your proposal checks out the following questions:
+
+- [ ] Is your feature request related to a problem? Please describe, *e.g.* "I'm always frustrated when..."
+- [ ] Describe (concisely) the solution you'd like.
+- [ ] Describe alternatives you've considered.
+- [ ] Do you have screenshots or sketches to illustrate your proposal?