Add minetest issue template (#6936)

* Add minetest issue template

This permits end user to have a basic template permitting to do issue triage and for users to know what we expect.

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,31 @@
+##### Issue type
+<!-- Pick one below and delete others -->
+ - Bug report
+ - Feature request
+ - Documentation issue
+ - Build issue
+
+##### Minetest version
+<!--
+Paste Minetest version between quotes below
+If you are on a devel version, please add git commit hash
+You can use `minetest --version` to find it.
+-->
+```
+
+```
+
+##### OS / Hardware
+<!-- General information about your hardware and operating system -->
+Operating system:
+CPU:
+
+<!-- For graphical issues only -->
+GPU model:
+OpenGL version:
+
+##### Summary
+<!-- Describe your problem here -->
+
+##### Steps to reproduce
+<!-- For bug reports or build issues, explain how the problem happened -->