Add issue templates (#3248)

Add issue templates of blank issue, improvement and reporting bug in WAMR.
And fix several invalid links in ATTRIBUTIONS.md.

.github/ISSUE_TEMPLATE/improvement.md

@@ -0,0 +1,28 @@
+---
+name: Improvement
+about: A feature request or code improvement.
+title: ''
+labels: ''
+assignees: ''
+---
+
+Thanks for filing a feature request! Please fill out the TODOs below.
+
+#### Feature
+
+TODO: Brief description of the feature/improvement you'd like to see in WAMR
+
+#### Benefit
+
+TODO: What is the value of adding this in WAMR? What problems does it solve?
+
+#### Implementation
+
+TODO: Do you have an implementation plan, and/or ideas for data structures or
+algorithms to use?
+
+#### Alternatives
+
+TODO: What are the alternative implementation approaches or alternative ways to
+solve the problem that this feature would solve? How do these alternatives
+compare to this proposal?