[Docs] Update issue templates

Author: wonniiii

.github/ISSUE_TEMPLATE/custom.md

@@ -0,0 +1,15 @@
+---
+name: Custom issue template
+about: Describe this issue template's purpose here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+## 🛠 Issue
+<!-- 이슈에 대해 간략하게 설명해주세요 -->
+
+## 📝 To-do
+<!-- 진행할 작업에 대해 적어주세요 -->
+- [ ] todo!