reworked GitHub issue templates (#2492)

.github/ISSUE_TEMPLATE.md → .github/ISSUE_TEMPLATE/BUG_REPORT.md

+---
+name: Bug Report 🐞
+about: Something isn't working as expected? Here is the right place to report.
+labels: bug
+---
+
 <!--
 Please search your question on previous issues, Stack Overflow (https://stackoverflow.com/questions/tagged/lightgbm) or other search engines before you open a new one.
 -->

.github/ISSUE_TEMPLATE/FEATURE_REQUEST.md

+---
+name: Feature Request 💡
+about: Suggest a new idea for the project.
+labels: enhancement, feature-request
+---
+
+<!--
+Please search your feature on previous issues and our feature requests consolidation hub (https://github.com/microsoft/LightGBM/issues/2302) before you open a new one.
+-->
+
+## Summary
+
+<!-- Briefly explain your feature proposal. -->
+
+## Motivation
+
+<!-- Why is it useful to have this feature in the LightGBM project? -->
+
+## Description
+
+<!-- Detailed description of the new feature. -->
+
+## References
+
+<!-- Any useful references, for instance, papers, implementations in other projects, draft code snippets, etc. -->