docs: git: replace feature request with feature design

We're at the point where we want to focus on simplifying
the codebase, so we change the mechanism of
proposing ideas into proposing solutions.

Author: shpaass

.github/ISSUE_TEMPLATE/feature_design.md

@@ -0,0 +1,9 @@
+---
+name: Feature Design
+about: Propose a feature that you want to implement in Yafc
+title: "[Feature Design] Your Title Goes There"
+---
+
+This text is a guideline. You can remove it before posting the issue.
+- A Feature Design is a proposal that describes the problem and provides one or more solutions to it. Use it if you are a developer and you want to add a feature to Yafc. 
+- The purpose of the Feature Design is to showcase how important solving the problem is to the playerbase, and describe the possible solutions to estimate how much complexity the feature would bring. Yafc aims to reduce the complexity of its codebase, so the less complexity the solution adds, the better.